Accelerator Pump Modifications

These modifications were initially used before tuning with leak jets and AP diaphragms became common. 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. One advantage to the BK mod is that it is adjustable "on-the-fly."

BK Mod



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):

1. 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.
2. 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!
3. 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.
4. 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

• 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.

HB/Doc Mod

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.

Procedure

1. Take a flat head screw driver and remove the leak jet.
2. Block the leak jet with something, so all of the fuel is pumped through the AP jet.  several ways to do this.  
   • Use a small ball bearing that just fits in the leak jet housing. A 1/8" ball bearing is perfect
   • 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.
   • 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.

3.                              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.