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:


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 dependant 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.
  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.
  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."
  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.
    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%.