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 it 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:

1. AP timing linkage. This controls when the squirt starts.
2. Leak jet size. This controls the volume of the AP squirt.
3. How quickly you twist the throttle. With a slow roll of the throttle, the AP is not activated.
4. 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.

For more information on AP function, see the Accelerator Pump 101 thread on ThumperTalk.

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).

For a discussion of accelerator pump modifications that can be made beyond normal tuning procedures see Free Mods: Accelerator Pump.

Leak Jet

Newer bikes (either 2001 and up or just 250Fs) have a leak jet that 'leaks' 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 2003-2004 models.

2. Adjust your idle speed to that recommended in the manual.

3. Adjust AP timing linkage (per manual)

4. Measure your AP squirt. Make sure the fuel 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 before hand, cracking the throttle numerous times to make sure it is well primed. Look into the carburetor using a flashlight. You will see the brass jet just behind the throttle plate on the bottom of the throat and slightly to the right of the middle. Quickly whack the throttle from closed to wide open. You will see a small stream of gas in the bottom of the venturi shooting back towards the slide. This is the AP squirt.  Now time the duration of the squirt.

   1. You can use a stopwatch to measure the time but given the very short duration this is likely somewhat inaccurate.

   2. Use a camcorder to record the squirt. Then replay the squirt frame by frame (30 frames/sec) and calculate the duration.

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).

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.

AP Diaphragm

There are also four different AP diaphragms available. 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 models' 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.

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. At immediate squirt from idle would be zero delay from the "touch point", where the rod just touches the diaphragm. A setting of 1/2 - 1 turn inwards delays it adequately to keep from squirting on the slide. This, I believe, was the .6mm measurement with feeler gauge. The original BK Mod posting from SUnruh also had the pump stroke set to 1 1/4 turns out from touch point to set the flow. This did not say the pitch of the screw being used and so was a little vague.

Throttle valve height

WR250F = 1.5 mm / 0.059 in

YZ250F = 1.25 mm / 0.049 in

Procedure

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.

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