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Tuning the Keihin Accelerator Pump


Rick_Kienle

Four stroke carburetors have accelerator pumps that produce a squirt only while the throttle is being added, not at a constant throttle or trailing throttle. The purpose is to make up the fuel lost to low vacuum until the revs build and recreate the vacuum. Like any other carburetor circuit (e.g., pilot circuit, needle circuit, or main jet circuit), there may be more fuel added or less fuel added than is needed. In addition, the AP squirt may be the right amount but not last long enough (to build revs) or may be too long (being rich after revs build).

 

The amount and duration of the AP squirt is dependent on a few things:

  • AP timing linkage. This controls when the squirt starts.
  • Leak jet size. This controls the volume of the AP squirt.
  • How quickly you twist the throttle. With a slow roll of the throttle, the AP is not activated.
  • Length of stub on the AP diaphragm. This controls the duration of the AP squirt.


When you twist the throttle, the AP linkage pushes a rod down against the AP diaphragm which pushes gas through a passage going to the carburetor venture. The squirt of gas compensates for the big gulp of air the carburetor sucked in. The sudden drop in vacuum causes less fuel to be sucked through the normal jets. To help in fine tuning this squirt, there are adjustments. The leak jet is like a bleed hole in the AP squirt passage. A slow twist of the throttle will push a small amount of gas through the leak jet (which is the path of least resistance) and almost none will make it through the whole passage into the venturi. If you twist the throttle quicker, it tries to force more fuel through the passage which can't all go through the leak jet, so the rest flows through the whole passage and squirts into the venturi. The timing screw (on the external linkage) lets you time when the squirt starts and to some extent the duration, i.e. earlier squirt equals slightly longer duration. The length of the rivet on the diaphragm will also control the duration. A longer rivet will cause the diaphragm to bottom out sooner and limit the travel of the diaphragm, therefore reducing the duration of the squirt. A shorter rivet will allow longer travel and therefore a longer squirt duration.

 


Leak%20Jet%203.jpg

 

The figure is a schematic sketch of the accelerator pump (AP) circuit. When the bike is running with the AP fuel reservoir full, and you whack the throttle, the actuator rod (green) gets depressed. This pushes the AP diaphragm (blue) down forcing fuel out the bottom passage. As you can see, the passage allows the gas to go in one of two directions. One passage leads directly to the AP nozzle in the carburetor venturi. The other passage leads to the Leak Jet (red) and back into the carburetor bowl.

 

A couple of things become immediately apparent. One is that by varying the size of the leak jet, we can vary the amount of fuel coming back to the bowl and, therefore the amount that goes to the AP nozzle. A larger leak jet allows more fuel back to the bowl, and less fuel to the AP nozzle. A smaller leak jet allows less fuel back to the bowl, and more to the AP nozzle. A such, a larger leak jet leads to shorter squirt duration/volume and a smaller leak jet leads longer squirt duration/volume.

 

Also, the rivet on the bottom of the AP diaphragm limits its travel. A longer rivet would equate to shorter squirt duration. Length of the AP diaphragm is measured, measured from the top of the AP diaphragm to the bottom of the rivet. Stock diaphragm length is 7.5 mm.

 

It is also important to set AP timing screw (on the left side of the carburetor, under the cover). If you set it too tightly, it partially depresses the actuator rod. This holds the AP diaphragm down, and reduces the volume of fuel in the AP fuel reservoir leading to shorter squirt duration/volume. If the AP timing screw is set too loose, the AP squirt lags the movement of the carburetor slide, also potentially producing a bog.

 

When the AP squirt is not correct in volume and duration, the bike will stumble and potentially stall with a quick blip of the throttle off of idle. Searching the list, and this site, I found that several other people had the same problem. Also, there appears to be no one fix that works on all bikes.

 

Especially on the 03 and later models (on which the BK mod is difficult or impossible), it is likely that the AP can be tuned pretty close using different leak jets, AP diaphragms, and adjusting the timing linkage (see below).

 

Approach to Curing the Bog

 

The big issue is that the same fix does not work across the board. There is no "quick" fix. This is because like all jetting, the AP function is dependent on multiple factors such as (but to a lesser degree than other jetting circuits):

  1. Riding style
  2. Temperature and elevation
  3. The other jetting circuits (primarily the pilot circuit but some on initial needle taper as well).
  4. Any mods/upgrades you have done, primarily those that affect intake or air flow such as carb mods, airbox mods, cam mods, porting, and exhaust.


So the only option you have is to meticulously work through the process until you get satisfactory results. Here are several things you can check and/or adjust that will affect AP function and the degree of off-idle bog you experience:

  1. Pilot circuit jetting. You must have your pilot jet, pilot air jet, and fuel screw all dialed correctly. One note here for WRF owners, the YZF has a larger pilot air jet, at least on the older models. I have a YZF pilot air jet in my WRF because I am running YZ exhaust cam timing, have opened my air box and have an aftermarket exhaust. Also dialing in your PJ and fuel screw settings with a tachometer is extremely beneficial to the process. Details can be found at https://web.archive.org/web/20150218162212/http://www.thumperfaq.com/jetting.htm#PJ
  2. Needle taper and clip position
  3. Idle speed, make sure your idle speed is correct. I run mine toward the high end or recommended range ~1900 rpm. Again, set this with a tachometer.
  4. AP linkage timing. The AP squirt needs to clear the raising of the slide. This can be done by visual adjustment. There are also a few ways of tuning this. One is in the manual, the other was reported in one of the magazines. Both methods are detailed at https://web.archive.org/web/20150218162212/http://www.thumperfaq.com/ap.htm
  5. Leak jet size. Again one size does not fit all. The correct size needs to be determined by timing your AP squirt and adjusting appropriately.
  6. AP diaphragm stud length. Again one size does not fit all. Also determined by timing the AP squirt. This is used in combination with the leak jet to further adjust squirt time. Very few have needed to make changes to the AP diaphragm. But keep in mine, this part wears out. So on older bikes it may need to be replaced.
  7. PowerNow. The powernow has some affect on the bog but is not a complete fix.
  8. P-38 Lightning. This is a bolt-on accelerator pump plate that supposedly cures the bog while adding horsepower and response. This product will also ease in starting. This product is basically a bolt on replacement for the stock AP cover on the bottom of the carburetor. You do have to use the OEM O-rings and screws. This item has a stud in the bottom of the chamber that limits travel of the AP diaphragm. There are also minor changes to the fuel ports. The same results can be achieved by using different AP diaphragms with different stud length. Some minor jetting changes may be required with this mod. My bike only required adjustment of the fuel screw. This did not completely cure my bog.
  9. Boyesen AP Cover. This is a new product that I just saw in the most recent issue of MXA. It is also a bolt on replacement AP cover but it uses a different approach than the P-38. This item moves the fuel passages to different locations within the reservoir. I do not have any information regarding the effectiveness of this part. If anyone has real world experience with this part please let us know.
  10. BK Mod. This is essentially a mod that allows control over the timing and volume of the AP squirt. This mod is easily applied to the 2001-2002 models and was the factory racing teams first approach to fixing the bog. It can be done on 2003+ models but it is slightly more difficult. The same results can be achieved with leak jet and diaphragm changes. The only real advantage to the BK mod is that it is more adjustable and can even be adjusted "on the fly." See https://web.archive.org/web/20150218162212/http://www.thumperfaq.com/ap_mods.htm
  11. The HB/Doc mod. This is essentially blocking the leak jet completely. If the #35 leak jet still produces too short a squirt, then blocking the leak jet may be necessary. See https://web.archive.org/web/20150218162212/http://www.thumperfaq.com/ap_mods.htm
    Also, please keep in mind this CANNOT be tested on the stand or with the bike in neutral. Even a properly tuned bike will stall or cough if the throttle is quickly twisted from closed to WOT if there is no load on the motor. The off-idle response MUST BE TESTED while riding under normal conditions.


Now, I clearly understand that all of this feels daunting to those who are uncomfortable with jetting and for those who have never delved into their accelerator pump. But it is really not rocket science once you start digging into it. The goal is to achieve an AP squirt of 0.5-1.0 seconds that just misses the slide. Also keep in mind, that for most it is quite easy to cure 90% of the bog with:

  1. Tuning the pilot and needle circuit
  2. Setting idle speed properly
  3. Adjusting the AP timing linkage
  4. Leak jet changes


Everything else listed above is for those who need that extra 10%.

 


Timing the AP Squirt Duration

 

Another source of frustration seems to be the process of accurately timing the AP squirt. This is quite essential to fixing your bog. This is best done by digital video but can be accomplished with analog video or with a stopwatch. Here is how to do it, step by step:

  1. Make sure the float bowl is full and that the pump diaphragm is loaded with gas. The best way to do this is to ride the bike around the block cracking the throttle numerous times to make sure it is well primed. When I am doing multiple measurements or testing different settings. I take the carb off the bike and place it in a vice. I then use a funnel to keep the float bowl full of gas.
    IMG_0299.jpg
  2. Remove seat, tank (but make sure carb float bowl is full of gas), rear fender, airbox/boot and subframe. If you have a powernow, it is easier to remove that as well.
  3. Use a flashlight to peer into the carb intake. You will see the slide in the closed position. Just in front of the slide (toward you) and just to the left of center is a small brass nipple that sticks up. This is your AP squirt passage.
  4. If you quickly twist the throttle you will see the slide move up rapidly and a stream of gas will emerge from the AP nozzle toward the motor. This is your AP squirt.
  5. Obtain the help of a friend or family member. My son is a good AP timing helper. Have your helper hold the flashlight on the carb intake and twist the throttle.
    • Hold a video camera or digital camera with video functions at an appropriate distance to allow visualization of the AP squirt as well as enough light to adequately see.
    • Observe the AP squirt "visually" and have a stop watch or stopwatch function on a wrist watch.

[*]Have your helper quickly twist the throttle form zero to WOT as fast as possible and hold it open for a few seconds. Either record the squirt or time it with the stopwatch. [*]Record or time multiple squirts (4-5) and average the results [*]Calculating the AP squirt

  • With a stopwatch, simply record the time and average the results.
  • If recorded with video go to you TV or computer (depending on video or analog). You must have the ability to review the tape in frame by frame mode. You must also know what the frame rate of the recorder is. Most analog and digital video cameras record at 30 frames/second. Some digital cameras with a video function record at 15 frames/second and some newer digital recorders record >30 frames/second.
  • Playback your recordings frame by frame. Find the frame in which the first appearance of the squirt is recorded. This is frame 1. Go through the recording frame by frame counting the number of frames the squirt is visible. Record this number for each of your recordings.
  • The calculation is simple # frames divided by #frames/second. For example, if your squirt lasted 18 frames and you were recording at 30 frames/second then the calculation would be 18/30 = a squirt time of 0.6 seconds. So this for each of the recordings and average your results.



Once you know what your current AP squirt time, you can determine which direction and by how much you need to adjust.

 


Leak Jet

 

Newer bikes (either 2001 and up or just 250Fs) have a leak jet that leak's some of the squirt back into the bowl. As indicated above, the leak jet gives you the flexibility to adjust the pump beyond the limits of those without it. The AP is purposely built too strong so a smaller leak jet would send most of the fuel into the venturi and a larger leak jet would send less into the venturi (that is, more would leak back into the bowl). This allows adjustment from too much to too little (volume). Part numbers for available leak jets can be found in the Yamaha Part Numbers section. The leak jets are numbered according to the size of the hole. For example, a #90 has a 0.90 mm diameter opening, a #80 has a 0.80 mm opening, and so on.

 

There was a Yamaha service bulletin in 2001 regarding the use of leak jets and AP diaphragms. The date of the Yamaha service bulletin is 8/24/01 and it is labeled: "Report Number: 01-002" "Models - YZ250~426F, WR250F~426F(All Years): Subject - Optional Accelerator Pump Diaphragms and Leak Jets." Most service departments should have it in a book on the shelf somewhere for those who are interested. There are two other service bulletins available in the links below.

 

Procedure

  1. Remove the rear fender, subframe, airbox, and airboot. I also take the bell of the carburetor. The bell is not removable on the 2003-2005 models.
  2. Adjust your idle speed to that recommended in the manual.
  3. Adjust AP timing linkage (per manual)
  4. Measure your AP squirt.
  5. Drop the bowl on the carburetor (you don't have to pull the carburetor). In the bottom of the bowl, about 1/2 way from the center to the back, brake side is a tiny brass jet with a flat head screwdriver slot in it. Unscrew it and see what number it has on it. Bigger number jets give less AP squirt (more is wasted back into bowl). Smaller number jets give more AP squirt (less is wasted back into bowl).
  6. The goal in adjusting the AP is to select enough squirt to get it past the low RPMs but not enough to outlast the low RPMs or create a too rich condition during the low RPMs. It will burble like a two stroke with the choke on if too much and just have low acceleration. It will cough, cut out, or die without bucking at all if too little. Experiment with different leak jet sizes, using the charts below as a starting point, until the bike runs the best and your AP squirt duration is in the desired range (~0.5-1 seconds, some say 0.4-0.8 seconds).


AP Squirt Times with Various Leak Jets (using the 2003-2013 WR/YZ 250F's standard diaphragm - 5JG-14940-76-00)

 


Leak Jet Duration (sec)
#90 0.25
#75 0.267
#70 0.50
#55 0.67-0.75
#50 0.467
#40 0.667
#35 0.8 - 1.13
Closed 1.4 - 1.733

 

Additional Observations

  • The spray nozzle in the venturi has a diameter of 0.3 mm which is significantly smaller than the leak jets in most cases. This makes leak jets very effective for setting your accelerator pump duration.
  • Do all of your testing on the bike, not on the stand. Warm it up by riding for at least 15 minutes before you judge the effectiveness of any changes.
  • If you are unable to get your squirt duration in the proper range, you might consider changing out the AP diaphragm (see below).
  • If you want to further tune your squirt duration, say somewhere between where the #50 and #60 leak jet puts you, try this. Install the #50 (longer squirt of the two), and back off the AP timing screw in ¼ turn increments, until you get exactly the squirt you want. This slightly depresses the actuator rod, lessening the gas volume, decreasing the squirt duration.
  • Don’t forget to have the rest of your jetting and carburetor adjustments in good shape. Especially the float level, pilot jetting, and main jet/needle combinations.


Yamaha OEM Part Numbers

 


YZF/WRF Leak Jet: 4JT-1494F-XX-00 (for all model years)

 

#135 XX = 34
#130 XX = 33
#125 XX = 32
#120 XX = 31
#115 XX = 30
#110 XX = 29
#105 XX = 28
#100 XX = 27
#95 XX = 25 (05 WR250F standard)
#90 XX = 23 (04-05 YZ250F standard)
#85 XX = 21
#80 XX = 19 (06-08 YZ250F standard
#70 XX = 15 (03-04, 06-09, 11-13 WR250F, 03 WR450F, 09-11 YZ250F standard)
#60 XX = 11 (01-02 WR250F, 04, 07-11 WR450F standard)
#55 XX = 09 (06-09 YZ450F standard)
#50 XX = 07 (05-06 WR450F standard)
#45 XX = 05
#40 XX = 03
#35 XX = 01

 

For finer tweaking, Sudco has Keihin leak jets from #35 to #140 in increments of 2-3.

 

Keihin part number: N424-52-xxx

 

Jet Size Sudco Part no.
#70 019.764
#68 019.763
#65 019.762
#62 019.761
#60 019.760
#58 019.759
#55 019.758
#52 019.757
#50 019.756
#48 019.755
#45 019.754
#42 019.753
#40 019.752
#38 019.751
#35 019.750

 

AP Diaphragm

 

There are also four different AP diaphragms available from Yamaha. Each diaphragm has a different length stub (rivet) on the bottom to bottom out on the pump cover. The measurement is from the top, where the rod contacts, to the bottom end of the rivet. Part numbers are located in the Yamaha Part Numbers section. Larger numbers will reduce the AP squirt duration by limiting the travel of the AP diaphragm, similar to the P-38. There was a revision to the 2002 model's upper dish to make them 1mm taller. This makes the rivet start closer to the bottom initially for a shorter stroke. This is likely why the BK mod does not seem to be necessary on 2002+ models.

 

On a YZ426F carburetor, the 5JG-14940-19-00 diaphragm squirts for about 1.3 seconds with closed leak jet. This is nearly 1/2 the flow of the -76-00 (standard on 250Fs), so all the above durations could be cut in half using the 5JG-14940-19-00 (9.0mm) diaphragm.

 

Additionally, the 2008 Honda CRF450R introduced a new diaphragm with a much shorter rivet (~4.83mm) & a longer rod. Along with the shorter diaphragm rivet, Honda introduced an updated accelerator pump cover that moves the check valve from the float bowl to the accelerator pump cover. Read more details at CRFsOnly.com

 

AP squirt Duration using OEM cover and #35 Leak Jet
Diaphragm / Rivet Size (mm) / AP Squirt Duration (sec)
5JG-14940-18-00 / 8.00 / 0.80 - 0.93 / DPH #25 (WR 450F standard)
5JG-14940-76-00 / 7.46 / 1.13 - 1.27 / DPH #30 (YZ/WR 250F standard)
5JG-14940-17-00 / 7.01 / 1.33 / DPH #35 (YZ 450F standard)
5JG-14940-16-00 / 5.96 / 1.47 / DPH #45

 

AP squirt Duration using OEM cover and blocked Leak Jet
Diaphragm / Rivet Size (mm) / AP Squirt Duration (sec)
5JG-14940-19-00 / 9.00 / 1.30 / DPH #15 (YZ 426F standard)
5JG-14940-18-00 / 8.00 / 1.40 / DPH #25 (WR 450F standard)
5JG-14940-76-00 / 7.46 / 2.07 / DPH #30 (YZ/WR 250F standard)
5JG-14940-17-00 / 7.01 / 3.03 / DPH #35 (YZ 450F standard)
5JG-14940-16-00 / 5.96 / 3.30 / DPH #45

 

AP squirt Duration using P-38 and #35 Leak Jet
Diaphragm / Rivet Size (mm) / AP Squirt Duration (sec)
5JG-14940-76-00 / 7.46 / 0.72 - 0.77 / DPH #30 (YZ/WR 250F standard)

 

Accelerator Pump Timing Adjustment

 

The other part that is often ignored is the adjustment screw that comes on the linkage stock. This is to set the starting point of the accelerator pump. Turning it in delays the start and turning it out advances the pump action to start earlier. An immediate squirt from idle would be zero delay from the "touch point", where the rod just touches the diaphragm. The linkage needs to be set with enough delay to keep the AP squirt from hitting the slide. Otherwise a "lean" bog would occur. There have been several methods discussed regarding setting this adjustment. Some say, simply turning the screw 1/2-1 turns out (from the all-the-way in position) is adequate. The OEM manual describes, in detail, the recommended method for setting the linkage. I do recommend visual inspection of the squirt and fine adjustment of this setting after using the manuals method. For unknown reasons, there are different "throttle valve heights" listed for the WRF and YZF.

 

Throttle valve height

  • YZ250F/YZ450F = 1.25 mm / 0.049 in
  • WR250F = 1.5 mm / 0.059 in
  • WR450F = 3.1 mm / 0.122 in


Procedure (refer to the images below)

  1. In order for the throttle valve height (a) to achieve the specified value, tuck under the throttle valve plate (1) the rod (2) or other suitable spacer with the proper outer diameter. (Note: the diameter of the spacer or rod should equal the throttle valve height listed above. I use a hex wrench of the proper measurement and make sure the flat sides are aligned correctly to give the proper height.)
  2. Fully turn in the accelerator pump adjusting screw
  3. Check that the link lever (4) has free play (b ) by pushing lightly on it
  4. Gradually turn out the adjusting screw while moving the link lever until it has no more free play


ap_timing_01.jpg ap_timing_02.jpg

 


Accelerator Pump Modifications

 

O-Ring Mod & Alternatives
The O-ring mod involves adding a #78 o-ring between the accelerator pump linkage & the throttle linkage. This will make the squirt stronger at the expense of a slightly shorter squirt. The disadvantage to the o-ring is that over time, the o-ring will stretch out. Some JD Jet kits will include 2 o-rings (a thick and thin one).

 

The second option is to wire tire the linkage together. While the wire tie won't wear out, you risk binding at WOT and breaking the plastic cam.
APWire-tie.jpg

 

The third option is a stiffer accelerator pump spring. They won't wear out and they won't bind, but they are the more expensive than safety wire or an o-ring.
tokyomodsAPS.jpg

 

Three companies make such springs:

 


The following modifications were initially used before tuning with leak jets and AP diaphragms became common (generally prior to 2003). Although these mods may still apply in certain situations, they are generally not necessary if you have tuned your AP squirt with the methods previously described. However, one advantage to the BK mod is that it is adjustable "on-the-fly."

 

BK Mod

 

bk_01.jpg

 

Similar to the way leak jets reduce the volume of the AP squirt, the BK mod reduces the duration of the squirt. Most of these bikes come with enough or too much and too long AP squirt. Larger leak jets and the BK mod control each.

 

The BK mod came with a suggestion of 0.3 seconds or a certain gap between the screw and the cam. This was suggested by Brian Kinney (Tim Ferry’s mechanic). You may have noticed that a top 5 kind of MX/SX rider like Ferry doesn't lug his bike around the track. He keeps the RPMs up, always picks an appropriate gear, and doesn't ride in the woods where the speed changes are more drastic. The 0.3 seconds may not be appropriate for the average rider. My guess is somewhere between .5 and 1 seconds is about right, depending on the caliber of rider, the type of track/woods, and if you keep it up off the bottom. At over maybe 6K RPMs, the AP is just wasting fuel.

 

When this mod is done correctly is will make your bike run and start better. It also will give quicker throttle response and eliminate the bog off the bottom. Your jetting will most likely need to be richened up since this mod shortens the duration of gas sprayed. Most agree that an AP squirt duration between 0.5 and 1 sec is optimum (see Tuning the AP Squirt). This mod was developed for the 2001 models. Many have suggested that this mod is not necessary on the 2002 models and it is difficult to do on the 03 models (see below).

 

Brian Kinney is Tim Ferry's factory mechanic (and the originator of the BK mod) and the following were his instructions.

 

Procedure (2001-2002 models):

  1. Drill and tap the pump cam stop and install a 4mm screw/spring combo. That will contact the pump cam. A hex socket bolt works well approx. 25mm long.
  2. Remove the subframe so you can look down the throat of the carb and with a stopwatch time the length of the pump spray when you stab the throttle. I usually click the stopwatch at exactly the same time I turn the throttle and click it again when I see it stop. It may seem weird but it works!
  3. Also you need to adjust the pump timing screw so that it does not hit the slide when it is rising. The timing is close when the spray just misses the slide.
  4. Then set the duration of the spray to .3 seconds with the adjustment screw you just installed. This may sound complicated but is the only way to get rid of the pesky bog off the bottom.


Procedure (2003+ models):

 


The 2003 carburetor was redesigned and as a result there is no place to drill the hole for the BK mod. The hole must be drilled in the throttle cover between the bottom mounting tab and the drain hose, there is not much room there but you can clean out a nice spot with a dremel tool to make room for the bolt. Drill the hole at a slight angle to get more surface of the bolt on the cam. Used a 6-32 tap and a 1" bolt and washer and a spring that fits the bolt. Then after you got it set to where you want it put some silicone on it to keep it in place, and to seal anything that my have been a problem around the threads.

 

bkmod_02.jpg

 

Helpful Tips from Motoman393

  • You don’t have to use a 4mm bolt he used a 6-32 1" long flat head screwdriver bolt. He used a spring out of a writing pen. His stock pump timing was around 4.5 secs and now it is around .35 sec.
  • When drilling put a piece of metal (like a hacksaw blade) in between where the drill bit will poke through and the black pump thingamajig.
  • The squirt of the gas was also off on his bike...it should spray gas so that it just clears the slide...but his sprayed when the slide was about halfway up the stroke.


HB/Doc Mod

 


This mod also allows you to control the timing of the AP squirt. Some have found (especially on newer bikes) that the AP squirt duration is actually too short. In which case, the BK mod is of no help. This mod increases the duration of the AP squirt. In most cases, the BK mod will be required afterwards to fine tune the duration.

 

Procedure

  1. Take a flat head screw driver and remove the leak jet.
  2. Block the leak jet with something, so all of the fuel is pumped through the AP jet. TT members have found several ways to do this.
    • Use a small ball bearing that just fits in the leak jet housing. TT member yzfmxer states that a 1/8" ball bearing is perfect
    • TT member vtss5000 found a bolt with the same thread and simply cut a section the same length as the jet, and cut a small slot into the end so i could use a blade screw driver to tighten it into place.
    • Many TT members solder the leak jet closed. Just make sure to get the jet good and hot with the iron, and be sure to use flux. Eventually the solder will flow into it. Then use a thin screwdriver to hog out the screw slot so you can install it.

[*]Replace the leak jet.


You will now have a squirt duration that is somewhere around 4 seconds. Four seconds is now way too long for an AP squirt duration. So now you need to do the BK mod. This way you will have total control of the pump duration. If the bike is jetted correctly before doing this mod, you might find that the jetting is too rich after the mod. Try one clip leaner on the needle, and down two sizes on the main jet. The pilot may need minor adjustment. You may also need to fine-tune your AP timing screw, because the strength of the squirt will be much stronger now. It has a mark on it, so try moving it either way until you can crack the throttle at idle without it stalling.

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      Why and how does detonation happen?
       
      Detonation occurs when the heat and pressure in the cylinder rise to the point where the gasoline is ignited by just that, and not by contact with a flame. Once that level of heat is reached, it affects all of the fuel that remains to be burned equally and at the same time, rather than progressively, as a flame traveling from droplet to droplet of the fuel mist, and all of it ignites in the same instant, creating insanely high spikes of pressure that can and do cause horrendous damage.
       
      In practice, the causes are one or more of excessive heat in the combustion chamber that allows temperatures to rise too high during compression, timing that is too far advanced, allowing the flame to build too much pressure before it can effectively turn the crank, hot spots in carbon deposits that ignite the fuel early, or simply a fuel the does not have sufficient resistance to being ignited by heat and pressure alone. That is, a fuel with an insufficient octane number.
       
      Most of the time, what happens is that ignition takes place normally, and the flame front begins to spread over the combustion chamber as intended. But, as the flame spreads, heat and pressures in the chamber rise until they reach the point where the fuel can no longer tolerate them, the remaining fuel from the previous intake cycle detonates, often resulting in an audible “ping”. It can produce a sound reminiscent of having filled the top end with marbles or something, and can be quite destructive if it happens early enough in the power stroke. The correction, assuming that the engine is timed and jetted appropriately is to increase the resistance of the gasoline to being ignited by sources other than open flame. That means, a fuel with a high enough octane number.
       
      That’s the form detonation usually takes, and at that level, in which only a part of the whole fuel charge is detonated instead of burned, it isn’t nearly as harmful as it can get. In the extreme, where the gasoline is pre-ignited, that is, ignited earlier than intended by an overheated engine, glowing carbon, etc., a larger percentage, or even all of the fuel may be detonated at once, and the results can be catastrophic.
       
      Up next: What are the factors that influence normal combustion?
       
       
      What are the factors that influence normal combustion?
       
      Compression, ignition advance, and the fuel’s tolerance of them. Compression is simple to understand. Compression creates heat, and more compression creates more heat. Possibly too much more.
       
      Ignition timing is a little more complicated, but still pretty easy to understand. In a piston engine, there is a mechanical “sweet zone” in the rotation of the crankshaft that goes from just after top dead center (TDC) to around 90 degrees after top dead center (ATDC). In this zone pressure on top of the piston does the most efficient job of turning the crank. It’s also important here to note that throughout this zone of rotation, the combustion chamber volume is constantly expanding. Gasoline of any kind or blend burns at nearly the same rate under pressure regardless of the situation.
       
      Now, picture the piston having just past TDC and starting down the bore. If you light the fuel now, it will take a certain amount of time for it to develop a significant amount of pressure to really do anything about pushing the piston down. While it’s trying to build pressure, the piston is moving away from it at the same time, and little is gained. So the timing is set to ignite the fuel in advance of TDC. This allows the burning fuel to build up a meaningful amount of “push” by the time the piston starts down, and ideally burning most of it within the “sweet zone”. Since the engine will speed up, but the fuel burn won’t, the faster the engine spins, the more advance it needs as it picks up speed.
       
      But if the fuel is ignited too early, the pressure and heat may reach critical levels before the combustion chamber volume has begun to enlarge adequately, and the portion of the fuel that lies in front of the advancing flame may then detonate.
       
      Octane is the fuel’s detonation resistance.
       
      And that’s all it is. Gasoline is a blend of several hydrocarbon solvents, among them toluene, benzene, heptane, and octane, plus a number of less active ingredients designed to do things other than add to the fuel’s energy levels. The number “100 octane” is based on the detonation resistance of 100% iso-octane. When a fuel is labeled “95 octane”, it resists detonation under pressure as well as a blend of gas consisting simply of 5% n-heptane and 95% iso-octane.
       
      Octane number indicates ONLY this resistance to detonation. High octane gas does not burn hotter, colder, easier, harder, cleaner, dirtier, or with any more or less power because of the octane number. Differences such as any of these other fuel characteristics that actually do occur are the result of the overall fuel blend used for that particular gasoline, and it is both possible and common to find major differences in these qualities in different gasolines that all have the same octane rating. Race gas is a perfect example of this, as we will see later.
       
      Up next: Why can I use 91 octane when my manual says I need at least 95?
       
       
      Why can I use 91 octane when my manual says I need at least 95?
       
      There are three different rating systems used to find the octane number of a fuel. The oldest is the Research Method. This method uses a special test engine with a variable compression ratio to compare the relative detonation resistance of fuels with equivalent heptane/octane mixes.
       
      A newer method called the Motor Octane method also uses a test engine, but runs at 900 RPM instead of 600 as in the Research Method, and uses higher temperatures and variable timing to compare fuels. It is considered a more accurate gauge of how gasoline will perform in modern engines than is the Research Method, but it’s rarely used in any kind of advertising because the rating numbers tend to run from about 8 to 12 points lower than the ratings arrived at with the Research engine. A fuel rated 100 Research Octane Number (RON) will only post up a best of 90-92 Motor Octane Number (MON), in spite of the fact that they have very close to the same real detonation resistance regardless of the test method. But oil companies are much more likely to promote their products by quoting RON than MON, if you let them, because it comports with all those marketing myths they’ve been selling all these years. This is where the third rating method comes in.
       
      In an effort to reduce consumer confusion and promote some level of consistency, the US Government requires that the average octane number achieved by both methods be posted on gas pumps and be called the “Anti-Knock Index”. You see it as “R+M/2” on the pump. So when your manual says you need 95 octane, and your bike is from Europe or Japan, you’re being quoted Research Octane Number. The equivalent Motor Octane number would be about 86, and the average would be 90-91, so that’s what you would look for at the gas pump.
       
      Up Next: So, do I need race fuel?
       
       
      So, do I need race fuel?
       
      If you can buy pump gasoline that meets the minimum octane requirements of your engine, you don’t need race gas or octane boosters to raise the octane number any higher. Your engine will run detonation-free on any gas that rises to that level, and paying any money out to run the octane rating up any higher than that is just a pure waste.
       
      There are, or may be, several other reasons to improve on the pump gas you find in your particular area. A lot of what goes into commercial automotive pump gas is there to do things other than create power, and those ingredients may be partially or completely inert as far as their contribution to the amount of power the engine can produce from it (referred to as “energy content”). Ethanol fuels are a good example. By itself, ethanol has an RON of 108, but its MON is only 88. E85 fuel is 104 RON, and only 85 MON. Furthermore, to get the same power as non ethanol gasoline, you have to burn 15-25% more of it.
       
      Oxygenating agents are added to pump fuels to aid in the more complete burning of fuel for the purpose of reducing emissions. Oxygenates are added to race fuels as accelerants, and there is often a fairly big difference in the chemicals chosen for that job. Ethanol is an oxygenate, but it produces much less energy per volume in and of itself than most gasoline components, so it reduces the energy content. MTBE (methyl tertiary butyl ether) is an oxygenate that produces more energy when burned than ethanol, and releases more oxygen in the process, so it’s more often used in race fuels.
       
      You can generally gain power through using race gas, but rather than a gift that keeps on giving, it’s a modification that you have to keep on paying for for as long as you want to use it, and it often requires rejetting to make the switch, so you kind of have to stay with it. The extra power IS NOT a result of the usually higher octane number, but comes from the specific blend of hydrocarbon compounds used in the formula. A number of octane increasing components such as xylene and toluene also increase the energy content since they actively contribute to the combustion event, as opposed to tetraethyl lead, with is essentially inert as a fuel component.
       
      As far as vaporization rates, burn rates, etc., etc., differently configured engines require fuels with different attributes. Factors are carbs vs. fuel injection, long vs. short intake tracts, high vs. low RPM operation, and steady state vs. non-steady state operation (like boats and airplanes vs. dirt bikes). Race fuels come in such diverse varieties for this reason.
       
      Incidentally, octane boosters are mostly snake oil. There are a few good ones that are available from automotive speed shops, but most of the ones you see on the shelf at the auto parts store are useless. They say they raise octane by one or two or three points, but that’s a change of 0.1 to 0.3, not 1 to 3 octane. Injector cleaner might actually be more effective.
       
      Next: So, will more octane benefit me?
       
       
      So, will more octane benefit me?
       
      An excess of octane number beyond what your engine needs is completely harmless and has no downside except for the damage it does to your wallet. If it’s simply a question of octane, as long as you don’t ping on ordinary pump premium, you don’t need any more octane to prevent detonation, and preventing detonation is the only thing high octane is good for.
       
      Have a question or comment? Post it below! You'll be helping to expand the depth & value of this topic and at the end of the day, that's my goal!
    • By Red_250X
      Just wanted to give you X fans some real world successful jetting settings. I just got back from a trip to Colorado and wanted to share these settings.
      Our camp was at 6100' and we rode up to 13,000'. Temperature was between 65 and 85 (absolutely PERFECT!).
      Modifications:
      Airbox opened about a third of CCC recommendations.
      Baffle removed, but spark arrester in place.
      Pilot jet changed from 40 to 42.
      Setup:
      -Needle in stock position
      -Main jet - 142
      -Air screw at stock position (I probably could have made some improvement, but I was having too much fun riding and I didn't experience any hesitation)
      The bike ran great and climbed some HUGE hills. Most of our riding was single track, switchbacks, very difficult and technical - a lot of 1st and 2nd gear.
      NOTE!! Apparently, the 250X is jetted lean in stock form. Opening the airbox and removing the baffle leans the carb even further. IF YOU DO THESE MODS, REJET YOUR CARB!!! Otherwise you could do serious damage to the motor. It will run significantly cooler when it is not lean. I live at about 600' and am running a 152 main jet.
    • By 230F
      Jetting the 230F
      By: Phil Vieira
      This project takes no less than 2 hours if you have never done jetting to a bike before. It took me 1.5 hours, to take my bike apart, take out the needle, change my pilot jet and the main, and take pictures along the way, but I have seen the inside of my carb 3 times, so I know my way around it pretty well…
      You should be jetting this bike right when you get it home. This bike comes lean from the factory. If you don’t know what that means, it means that the bike is getting too much air, in terms, a hotter engine, and your plugs will get hotter, and a decrease in HP. To make your engine last longer, do this.
      These jetting combos are for a 2000 feet and below scenario. Any altitudes higher, you should do a search on the forum. If it cannot be found, post on the forum. Please don’t post on the forum “How do I do this…” You have all the answers here.
      This project comes to a grand total of less than 30 dollars. The needle is 20, the main jet is about 3 dollars, and the pilot is 5 dollars. You may not need to do the pilot jet depending on your situation, but again, if you’re riding 2000 feet and below, it’s a good idea to get a pilot jet.
      The jets I used consist of a 132 main, 45 pilot and the power up needle with the clip on the 4th position.
      Part numbers:
      16012-KPS-921 – Needle (Includes Power up needle, Clip, and needle jet)
      99113-GHB-XXX0 – Main jet (Where XXX is the size)
      99103-MT2-0XX0 – Pilot jet (Where XX is the size)
      For the Jets, just tell them you need jets for a regular Keihn carb, (also known as a Keihn Long Hex) main jet size XXX, pilot jet size XX. They should know the part numbers. For the needle, bring the number along. If you are lazy, they should have a fiche and they can look up the numbers. Then again you can take in the old jets, and make sure they match up to the new ones.

      Now, the tools you will need are as follows:

      ~A collecting cup of some sort. I used a peanut butter jar.
      ~Ratchets for the following sizes:
      - 6mm, 8mm, 10mm, 12mm
      - Extension for the sockets needed
      ~Phillips and Flathead screwdriver (Be sure these are in perfect condition. A badly worn screwdriver will strip the screws)
      ~Needle nose pliers
      ~”Vise grips” or known as locking pliers (Two)
      ~Open end wrench 7mm and 12mm
      ~ It’s a good idea to have a extra hand around
      (Not needed, but I highly recommend tiny Phillips and flathead screwdrivers (Pictured next to the jar and the ¼” extension) I recommend these for removing a couple things since you can put pressure with your thumb on the end and unscrew it with the other hand. This insures that you will not over tighten any parts, and ensure that you will not strip the heads of the bolts.
      Ok, now that you have the tools, let’s start by putting the bike on a bike stand. I put it on the stand rather than the kickstand because it’s more stable and sits higher. I hate working on my knees. Start by taking the number plates off. Yes, both of them. The right side, you take off one bolt and the top comes off of its rubber grommets, pull the top off, and the plate comes right off. The left hand side, use the 10mm socket to take the battery bolts off, and then take the Phillips bolt near the back. Again, rubber grommets are used to hold the top in place. Take the seat off. There are two mounting bolts on the back:

      Those two bolts are both a 12mm socket. Use the open end wrench on the inside, and use the socket on the outside. You may need to use an extension if you don’t have a deep socket. Once you have the two bolts off, slide the seat back, and lift it up. This is what you have. Notice there is a hook in the middle and a knob on the tank. That is what you are sliding the seat off of.

      Now that the seat is off, you must take the gas tank off. Don’t worry, you won’t spill any gas any where, I promise. On the left hand side of the bike where the valve is, slide down the metal clip holding the tube in place. Turn off the gas supply, and slip the tube off slowly. Now take off the two bolts in the front of the take. This is on the lowest part of the gas tank in the front, behind the tank shrouds. The socket you will use is an 8mm socket. Take the bolts all the way off and set them aside. Now look back at the last picture posted. On the back of the tank, there is a rubber piece connected to the knob and the frame. Slip that rubber piece off of the frame. Pull the vent tube out of the steering stem and lift the tank up. Don’t tip it, and lay the tank aside where you won’t trip on it. This is what you’ll end up with:

      It may be a good idea to take a rag, and wipe all the dirt off the top of the bike if any. You don’t want anything dropping down into the carb. If you do, engine damage is the result. A clean bike is always a good thing! Now we must drain the gas out into that container. This is very easy. Make sure you open the garage door, windows, whatever, to let the fumes out. Breathing this crap is bad. Here is where the drain screw is:

      (Don’t worry about removing the carb, that comes later) This is on the right side of the carb, on the float bowl. The vent tube that goes down to the bottom of the bike is where the gas drains to. Put the jar under that tube and start to unscrew that screw, enough so that the gas leaks into that jar. Once the gas doesn’t drip anymore, close the screw all the way. Now on to the top of the carb. We are going to take this cover off:

      This cover comes off by removing the two screws. Once removed, the lid comes off as well as the gasket. Flip it over and set it aside. Do not set the gasket side down on the ground, as it will get contaminants! Here is what you are facing:

      The angle of the camera cannot show the two screws. But one is visible. It has a red dot, and opposite of that side is a darker red dot. I made it darker because it’s not visible, but that is where it is. This is where I use the miniature screw drivers to get the screws. I magnetize the screwdrivers, and use care to make sure I don’t strip the heads. Metal pieces in a piston are not good! Remove the two screws. Put these screws on a clean surface so they do not get contaminants. Now get your vise grips and set it so that it will lock onto the throttle, not too tight, not too loose. Set the vise grips on the seat. Start to open the throttle slowly as you guide that “plunger holder” (as I call it) up to the top. Once you have the throttle all the way open, take the vise grips, and lock it so that the throttle does not go back any more. What I do is I hold it pinned and lock it up against the brake so it doesn’t rewind on me. If you don’t have locking grips, a friend will do, just have them hold the throttle open all the way until you are finished. How fold the plunger holder to the back of the carb and pull the piece up to the top. Take care not to remove it, as it is a pain to get back together! If it came apart on you, this is what it should be assembled to:

      Once you get the holder out of the slider, set it back like this:

      As you can see, the bar is back 45 degrees, while the holder is forward 45 degrees to make a S. Here is what you are faced with when you look down on the carb:

      Where the red dot is where the needle lies. Grab needle nose pliers and carefully pull up the needle out of its slot. This is what the needle looks like once it is out.

      Now we must move the carb to take the bowl off. Untie the two straps on the front and back of the carb. Don’t take them off; just loosen them until the threads are at the end. Take the front of the carb off the boot and twist the bowl as much as you can towards you. Tie the back tie down to that it does not rewind back on you. This is what you have:

      Now we must take off the bowl. Some people take that hex nut off to change the main jet, which you can, but you cannot access the pilot jet, and you can’t take out the needle jet (a piece the needle slides into), so we need to take it off. It’s just three bolts. As we look at the underside of the carb, this is what you will see:

      The bolts with the red square dots are the bolts you will be removing. These are Phillips head bolts, and the bolt with the blue dot is your fuel screw. This is what you will adjust when the time comes, but keep in mind where that bolt is. You need a small flat blade to adjust it.
      Well, take those screws off, and you are faced with this:

      The blue dot is for cross reference, which is the fuel screw once again. The green dot is the pilot jet. You can remove this using a flat blade screwdriver. Just unscrew it and pull it out. Once you pull it out, set it aside and put in the 45 pilot jet you got. The red dot is the main. You remove this by using a 6mm socket. Just unscrew it. If the whole thing turns, not just the jet, but the 7mm sized socket under it, don’t worry, that piece has to come out as well. If it doesn’t, use a 7mm to unscrew it off. Here is what the jets look like:

      Pilot Jet

      Main jet attached to the tube. Take the main jet off by using an open end wrench and a socket on the jet. Again, it screws right off.
      Here is what you are faced with if you look form the bottom up.

      From left to right: Main jet, Pilot Jet, Fuel screw. Now in the main jet’s hole, if you look closely, you see a bronze piece in the middle of that hole. We are going to take this off. Since I did not do this part (I only changed my pilot jet when I took these pictures) there are no pictures taken for this section but this is really simple to do if you’ve been a good student and know where things go. You should know anyways, you have to put the bike back together!
      (Notice: There have been discussions about these needle jets being the same. Only change this needle jet if the one you have is worn out. If you do not have the old needle, a older drill bit bigger than 3/20ths (.150), and smaller than 11/100 (.11") Use the tapered side of the bit, set it down in the hole and tap it out carefully.)
      Now take your OLD needle, I repeat, the OLD needle because what you are going to do next will ruin it. Pull the clip off with your needle nose pliers, or a tiny screwdriver to pry it off. Then put the needle back in the hole where it goes. That’s right, just to clarify, you took off the needle, and you put the needle back in the hole with no clip. Slide the point side first, just as it would go normally. Now if you look at the bottom of the carb, the needle is protruding past the main jets hole. Grab another pair of locking pliers (vise grips as I call them) and lock it as tight as you can on the needle. Pull with all your might on the needle. Use two hands. Have a friend hold the carb so you don’t pull it off the boot. Tell them to stick their fingers in the hole that goes to the engine, and pull up. After pulling hard, the needle jet should slip right off. Then notice which side goes towards the top of the carb. There is one side that is a smaller diameter than the other. Take the new needle jet, and push it up into the hole the way the old one was set. Just get it straight. Take the tube the main jet goes into, and start threading it in. Once you can’t tie it down anymore with a ratchet, unscrew it and look at the needle jet to make sure it’s set. That’s it for the needle jet. Now let’s start putting the carb back together.
      (Notice: Many people have destroyed jets and such by overtighting them! Use the thumb on the head of the wrench and two fingers on the wrench to tighten it down.)
      Thread the main jet into the tube it goes into, and then start putting it back on the carb. Thread the pilot jet in as well if you haven’t done so already. Remember these carburetor metals are soft as cheese, so don’t over tighten the jets very much. What I do is I put my thumb on the top of my ratchet, and use two fingers closest to the head of the ratchet to tighten the jet. That’s how tight I go when I tie them back in.
      Now before we put the carb back together, let’s adjust the fuel screw. Take a small screwdriver, and start screwing in the fuel screw until it sets. Again, do not over tighten, just let it set. Then count back your turns. Count back 1.75 turns.
      Now we must put the bowl back on. The white piece that came off with the bowl goes back as followed:

      If you look directly under the carb, the round hole is aligned with the pilot jet. Take the float bowl, and put it back on.
      Untie the rear clamp and the front clamp as well. Slip the carb back the way it used to. Make sure that it is straight up and down with the rest of the bike. The notch on the front boot should be aligned with the notch on the carburetor, and the notch on the carburetor should be in that slot. Tie the clamps down securely.
      Let’s put the needle in. These are how the needle numbers go:

      The top clip position is #1, the lowest one, closest to the bottom, is #5. (The picture says six but it is five in this case) For reference #1 is the leanest position, while 5 is the richest. I put the clip in the 4th position. Read at the bottom of the page and you can know what conditions I ride in, and you can adjust them to your preference.
      Put the clip in the new needle, slip it in. Take the vise grips off your grips and start guiding the plunger holder down to the bottom. Remember not to let that assembly come apart because it is a pain in the ass to get it back together! Once you get it to the bottom, put the two screws on, and then put the cover on.
      Now that you have done the carburetor mods, there is still one thing you want to do to complete the process. Don’t worry, this takes less than a minute! On the top of the air box there is a snorkel:

      As you can see, you can slip your fingers in and pull it out. Do that. This lets more air in to the air box. Don’t worry about water getting in. There is a lip that is about 1/8” high that doesn’t let water in. When you wash, don’t spray a lot under the seat, but don’t worry about it too much.
      The next thing you must do is remove the exhaust baffle. The screw is a torx type, or you can carefully use an allen wrench and take care not to strip it:

      The screw is at the 5 o’clock position and all you do is unscrew it, reach in, and yank it out. This setup still passes the dB test. The bike runs 92 dB per AMA standards, which is acceptable. Just carry this baffle in your gear bag if the ranger is a jerk off. I’ve never had a problem, but don’t take chances.
      That’s it! Start putting your tank on, seat, and covers. After you put the seat on, pull up on the front, and the middle of the seat to make sure the hooks set in place.
      Turn on the bike, and take a can of WD-40. Spray the WD-40 around the boot where it meets the carburetor. If the RPM rises, you know you have a leak, and the leak must be stopped. You must do this to make sure there are no leaks!
      Here is my configuration:
      04’ 230F
      Uni Air filter
      132 Main Jet
      45 Pilot Jet
      Power up needle, 4th clip position
      Fuel screw 1.75 turns out
      Riding elevation: 2000ft - Sea level
      Temperature – Around 60-90 degrees
      Spark Plug Tips
      When you jet your carb, a spark plug is a best friend. Make sure your spark plug is gapped correctly, (.035) but that’s not all that matters. You want to make sure the electrode is over the center, and you want the electrode to be parallel, not like a wave of a sea. Put in the plug, and run the bike for 15 mins, ride it around too then turn it off. Then take off the spark plug after letting the bike cool. The ceramic insulator should be tan, like a paper bag. If it is black, it is running rich, if it is white, it is running lean. The fuel screw should be turned out if it is running lean, and turned in if it is running rich. Go ¼ turns at a time until your plug is a nice tan color.
      Making sure your bike is jetted correctly
      While you are running the bike for those 15 mins to check the plug color, you want to make sure it’s jetted correctly now. Here is what the jets/needle/screw control:
      0- 3/8 throttle – Pilot jet
      ¼ to ¾ throttle – Needle
      5/8 – full throttle – Main jet
      0-Full – Fuel screw
      Pin the gas, does it bog much? Just put around, is it responsive? When you’re coming down a hill, the rpm’s are high and you have no hand on the throttle, does it pop? If it pops, it is lean and the pilot jet should be bigger. If it’s responsive your needle is set perfectly. You shouldn’t have to go any leaner than the 3rd position, but I put mine in the 4th position to get the most response. Your bike shouldn’t bog much when you have it pinned. If it does it is too rich of a main jet.
      Determining the plug color, you will have to mess with the fuel screw.
      That’s it, have fun jetting, and any questions, post on the forum, but remember to do a search first.
      Also, if your bike requires different jets due to alititude, humidity, or temperature, please post the following so we can better assist you:
      Average temperature
      Altitude (If you do not know this, there is a link in the Jetting forum that you can look up your alititude)
      Average Humidity
      What jets you are currently running
      What the problem is (If there is one)
      Just do that and we'll help you out the best we can.
      EDIT: The girl using this login name is my girlfriend. You can reach me on my new login name at 250Thumpher
      Then again, you're more than welcome to say hi to her!
      -Phill Vieira