Info:
This accel pump mod is ONLY for the 00'-02' YZ426, 01'-02' YZ/WR250F and 00'-02' WR400/426) this WILL NOT work on 98-99 YZ400's and 99 WR400's. 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 (from 1.7 secs stock to .3 secs)

The Procedure that Brian Kinney gave me:

• Brian Kinney is Tim Ferry's Factory mechanic (and the originator of the BK mod) and these were his instructions...To limit the accel pump spray length you must 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. 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! Also you need to adjust the pump timing screw so that it does not hit the slide when it is raising. The timing is close when the spray just misses the slide. Then set the duration of the spray to .3 seconds with the adjustment screw you just installed. This may sound comlicated but is the only way to get rid of the pesky bog off the bottom.

Helpful Tips from me:

• You dont have to use a 4mm bolt I used a 6-32 1" long flat head screwdriver bolt. I also used a spring out of a pen that you write with. My stock pump timing was around 4.5 secs and now it is around .35 secs (maybe that was why is flooded easy and was sometimes hard to start when hot?) Also when drilling put a piece of metal (like a hacksaw blade) inbetween where the drill bit will poke through and the black pump thingamajig LOL. The squirt of the gas was also off on my bike...it should spray gas so that it just clears the slide...but mine sprayed when the slide was about halfway up the stroke!
  
  Just by starting up the bike I could literally "feel" the difference, it felt like the snap of a 250 2 stroke and didnt have any bog whatsoever! You will really be amazed by what $4 does to the performance of this bike! The jetting I ended up with: 168 main, 42 pilot (1 3/4 turns out), needle in middle position.