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.

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.

6.                              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.

7.                              Record or time multiple squirts (4-5) and average the results

8.                              Calculating the AP squirt

o                                 

o                                With a stopwatch, simply record the time and average the results.

o                                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.

o                                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.

o                                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 standard diaphragm)

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)

#130 XX = 33
#120 XX = 31
#110 XX = 29
#100 XX = 27
#90 XX = 23
#80 XX = 19
#70 XX = 15
#60 XX = 11
#50 XX = 07
#40 XX = 03
#35 XX = 01

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 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 -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.

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
5JG-14940-76-00 / (standard) / 7.46 / 1.13 - 1.27
5JG-14940-17-00 / 7.01 / 1.33
5JG-14940-16-00 / 5.96 / 1.47

AP squirt Duration using OEM cover and blocked Leak Jet
Diaphragm / Rivet Size (mm) / AP Squirt Duration (sec)
5JG-14940-18-00 / 8.00 / 1.40
5JG-14940-76-00 / (standard) 7.46 / 2.07
5JG-14940-17-00 / 7.01 / 3.03
5JG-14940-16-00 / 5.96 / 3.30

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

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

• WR250F = 1.5 mm / 0.059 in
• YZ250F = 1.25 mm / 0.049 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