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.
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):
- Riding style
- Temperature and elevation
- The other jetting circuits (primarily the pilot circuit but some on initial needle taper as well).
- 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:
- 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
- Needle taper and clip position
- 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.
- 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
- 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.
- 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.
- PowerNow. The powernow has some affect on the bog but is not a complete fix.
- 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.
- 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.
- 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
- 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:
- Tuning the pilot and needle circuit
- Setting idle speed properly
- Adjusting the AP timing linkage
- 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:
- 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.
- 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.
- 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.
- 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.
- 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.
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.
- 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.
- Adjust your idle speed to that recommended in the manual.
- Adjust AP timing linkage (per manual)
- Measure your AP squirt.
- 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).
- 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)
#35 0.8 - 1.13
Closed 1.4 - 1.733
- 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.
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)
- 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.)
- Fully turn in the accelerator pump adjusting screw
- Check that the link lever (4) has free play (b ) by pushing lightly on it
- Gradually turn out the adjusting screw while moving the link lever until it has no more free play
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).
Three companies make such springs:
- Merge Racing #00-018 (potentially discontinued) (Installation instructions)
- R&D Racing
- Tokyo Mods #3901
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."
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):
- 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.
- 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!
- 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.
- 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.
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.
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.
- Take a flat head screw driver and remove the leak jet.
- 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.