Fixing the off-idle bog.

Fortunately those of us who have come before have figured out how to solve this problem. The issue is that the same fix does not work across the board. This is because like all jetting, the AP function is dependant on multiple factors such as:

1) Riding style

2) temperature and elevation

3) the other jetting circuits (primarily the pilot circuit)

4) and 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 and www.thumperfaq.com/jetting.htm#PJ

2) Idle speed, make sure your idle speed is correct. I run mine towards the high end or recommended range ~1900 rpm. Again, set this with a tachometer.

3) 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 www.thumperfaq.com/ap.htm

4) 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.

5) 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.

6) PowerNow. The powernow has some affect on the bog but is not a complete fix.

7) 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 acheived 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.

8) 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.

9) BK Mod. This is esstially 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 resuls can be acheived 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 www.thumperfaq.com/ap_mods.htm

10) 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 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 qucikly 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.

All this information can be found in more detail at:

www.thumperfaq.com/ap.htm

www.thumperfaq.com/ap_mods.htm

and the links provided on these pages.

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. Also keep in mind, that for most it is quite easy to cure 90% of the bog with:

1) Tuning the pilot circuit

2) Set idle properly

3) Time AP linkage

4) Use proper leak jet

The goal is to acheive an AP squirt of 0.5-1.0 seconds that just misses the slide.

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

SUnruh, can you make this a sticky!!!!!

rick,

excellent summary.

jim aka the wrooster

'01 wr250f

ps: i don't think making it sticky is going to help anyone find it. ?

Thank you Rick ?

now i gotta just find a time when i actually want to go through and read all of it ?

Another source of frustration seems to be the process of timing the AP squirt. This is quite essential to fixing you bog. Here is how to do it, step by step:

1. 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.
   
2. 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 tot he left of center is a small brass nipple that sticks up. This is your AP squirt passage.
   
3. 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.
   
4. 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.
   1. 
      
   2. 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.
      
   3. Observe the AP squirt "visually" and have a stop watch or stopwatch function on a wrist watch.
      
   4. Have your helper quickly twist the throttle form zero to WOT as fast as possible and hold if open for a few seconds. Either record the squirt or time it with the stopwatch.
      
   5. Record or time multiple squirts (4-5) and average the results.
      

[*]Calculating the AP squirt

1. With a stopwatch, simply record the time and average the results.
   
2. 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.
   
3. 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.
   
4. 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.

Accessing the jets needed to tune the AP squirt:

1. Turn the fuel petcock OFF
   
2. Open the float bowl drain tube and allow the gas to drain out
   
3. Take off the 17mm hex nut at the very bottom of the carburetor. A small amount of gas will pour out that is in the nut.
   
4. If you feel in the middle of the hole where you pulled the nut from, you will feel the small hex nut of the MJ.
   
5. The pilot jet is in front of the main jet and has a flat screwdriver head. Simply use a small short screwdriver to remove it.
   
6. The needle is under the very top of the carburetor. You will need to remove the tank and seat. There are two 3mm hex bolts on the front and back of the top cover. Clean off the top of the carburetor and the bottom of the frame above the carburetor with mineral spirits or WD40 and a rag. Take off the two hex bolts. Carefully remove the top of the carburetor. It has a rubber gasket attached so try to do this without rubbing it around too much. Looking down into the carburetor you will see the throttle slide. Twist the throttle and it will rise. In the middle of the throttle slide is a 4mm hex screw. Remove it and it will come out along with a spring and a collar. Don't loose the spring and collar. Under it is the round top of the needle. Using tweezers or needle nose pliers, gently lift it. Have someone twist the throttle to raise it up if you have trouble getting to it. Pull it out.
   
7. The leak jet is located in the float bowl. You will need to remove the float bowl and then use a small screwdriver to remove the jet.
   
8. The pilot air jet is the intake port of the carburetor. You will need to remove the air boot and the intake bell to access them. You may need to take the carburetor off for the air jets. Use a small screwdriver to remove the jets.
   
9. The fuel screw is at the bottom of the carburetor in the front (engine side) recessed in a small hole cast into the float bowl. You can't see it and you probably don't have a short enough screwdriver narrow enough to get in that hole. You will either need a special fuel screw tool or better yet put in a quick adjust fuel screw.
   

Here is how you can deterimine if you have the appropriate pilot jet size and how to set your fuel screw:

When the pilot circuit is jetted properly, starting the bike should not be a problem. You will need the choke to start cold and it won't run well off of choke for 30 seconds or so. When hot you will probably need the hot start. The bike should idles smoothly and have no hesitation of idle and minimal backfiring on deceleration.

Setting the Fuel Screw / Pilot Jet "by Ear"

1. Adjust the idle with the black knob until it is too fast. Then adjust it back down until it is around 1900-2000 RPM or if you don't have a tachometer (see below) until it sounds just a little high.
   
2. Before you start adjusting, count the turns required to tighten it up lightly.
   
3. Then start the bike with the slightly elevated idle and turn it out 1/4 turn, 1/2 turn, 3/4 turn and so on until you get to 2 turns. Listen for best RPM and best response to a quick 1/4 turn tweak of the throttle at each position of the fuel screw.
   
4. Now turn back in 1/4 turn at a time doing the same thing. By now you should have been able to distinguish the speed of the idle and the responsiveness to tweaking the throttle.
   
5. If it gets better between 3/4 and 2 turns out, set it at the best location and leave the rest of the pilot circuit alone.
   
6. If it is getting better turning it in or is best less than 3/4 turns out, replace the pilot jet with a smaller one and go through this procedure again.
   
7. If it is getting better as you turn it out or best at more than 2 turns out, replace the pilot jet with a larger one and go through this procedure again.
   

Setting Fuel Screw / Pilot Jet with Tachometer

1. Warm the bike up by riding about 10 minutes. Place it on a stand, have it idling. If you have a fan, direct it into the radiators (A YZF will start to boil out if you take too long to do this, WRFs have a nice catch tank).
   
2. Turn the fuel screw 1 1/2 turns out. Read the RPM for about 10 seconds (on my tachometer, cause it bounces around). If the avg RPM is not between 1700 and 1900, adjust to about 1800 with idle screw knob on carburetor Write down average RPM.
   
3. Turn the fuel screw 1/2 turn out. Write down the average RPM.
   
4. Turn the screw 3 turns out. Write down the average RPM.
   1. 
      
   2. If #2 is greater than #3 or #4, you have the right pilot jet. Usually the difference will only be 50 to 100 RPM. Go on to step #7.
      
   3. If #3 is greatest, you need less fuel. Install the next smallest number pilot jet. Go to step #3 and repeat.
      
   4. If #4 is greatest, you need more fuel. Install the next largest number pilot jet. Go to step #3 and repeat.
      

[*]Adjust the fuel screw in 1/4 turn increments around 1 1/2 turns out and find the maximum RPM fuel screw position. If the idle is now above 1900 RPM, turn it down to be in spec.

[*]If you get a little deceleration backfiring on closed throttle, try adding another 1/4 turn out. Remember you will need to redo this if the altitude or temperature changes significantly.

Induction type tachometers are available for ~$30-40 from SenDec or other sources. These are the most accurate. Vibration type tachometers can be purchased for less than $10 from most small engine (ie lawnmower) repair shops. These are more difficult to use and are less accurate.

As usual Rick, a most excellent write up. I recognize some of it from the faq. Those parts that you just wrote up should be appended to the faq.

Excellent articles Rick!? :cry: ?

I just checked mine .9 sec ... i guess it is a little rich. Thanks for the great info.

Great writeup! Just what I'm looking for.

Once I time my AP squirt, how do I adjust/alter it?

TomJV