accelerator pump adjustment - BEWARE!


If you are just interested in the results and don't want to waste time with my rather long story, just jump to the end of the msg for the conclusion.

There has been the suggestion posted on this forum, that you should adjust the activating pump lever arm so it floats in the middle of actuating arm hole - don't do this. Read the following story to see which effects this can have!

It all started when I rebuild my 1998 WR400 this winter. The bike is already more a YZ with WR gears and ignition. While at it, I also fabricated a new exhaust from a left over Big Gun for my last Husaberg. The bike ran really well, except for this long straight on my home MX track. After about 2-3 seconds of full throttle the bike started to cut out (actually it was more a little stumble) and only after another 2-3 seconds it would accelerate again. Otherwise there were no problems, the jetting seemed to be fine and with the new exhaust the bike had gained a lot of low end snap (much better than with my WB E-series), so I left it alone. The troubles started when I switched back to the WB e-series with a quiet-insert for a local enduro. The enduro went fine (bikewise ;-), but as soon as I went to the MX track, after about 3 fast laps the bike would start to backfire heavily and finally die - fouled plug! The carb was taken apart and cleaned, I went to a leaner jetting and I removed the insert. Still the same problem, a few fast laps on the MX track and the bike died. Let it cool down and it would run fine again, do this a few more times and the plug would quit working. I was pretty sure that the jetting was ok, because I had run in exactly this configuration for the whole last year with the same plug. To make things worse, the bike would run fine for and hour in an 1st and 2nd gear (going around tires, practise corners, ...) training session, but as I went onto the MX track for some fast laps during a break, the bike would die again. I was pretty sure now, that there must be something wrong with the ignition and so I swapped the CDI and ignition coil - still no go, bike dies after a few laps. In a last-resort-attempt before going deeper into the engine I disconnected the accelerate pump (AP from now on) and suddenly the bike ran fine, though hesitating when opening the throttle too fast (like it should be)!!! What the f**k was going on here.

I had done nothing to the AP this winter, except for cleaning and replacing the rod because it was corroded. Fortunately I could borrow a FCR carb from a buddies 1998 WR and through comparison I discovered that my AP was injecting almost TWICE the amount of fuel like his! OK, the amount of fuel injected is defined ONLY by the stroke of the AP rod and size of the AP body. While the body size is fix, the stroke is defined by its starting and end point. The end point is fixed because of a stop on the diaphragm, but the starting point is variable and defined through the position of the rod at idle. It turned out, that the new rod I had installed was 0,3mm (3 tenth of a Millimeter) shorter than my old rod and the rod in my buddies carb. Because of the shorter rod the starting point and therefor stroke of the AP increased, DOUBLING the injected amount of fuel. This only mattered, when going WFO for more then 2-3 seconds (time needed to empty the AP), but not keeping the throttle WFO long enough to burn the excess fuel and repeating this often - exactly the conditions you have on a outdoor MX-track!

Solution: I shortened the stroke by compressing the fork shaped stop behind the arm stop on the actuating lever. But keep in mind, only a very little adjustment was needed here to reduce the injected amount by half - SO DON'T MESS WITH THIS!

Conclusion: I would think that Keihin or Yamaha test each carb to make sure that the AP is working properly - no carb is the same. When replacing parts, like the rod, tolerances can easily cause problems and things can get even worse when you start 'adjusting' things!

Michael, happy that his 'baby' is fine again!

This is exactly what the P-38 lightning accelarator pump (AP) chamber does it shortens the stroke of the AP. The adjustment of the AP is almost a individual thing on a carb by carb basis. I too have noticed the symptoms you have encounter during testing and in high speed trail riding situations. The AP adjustment is still part of the tuning tool kit available to each of us, you must understand what the imapact of each tuning action you take on your ride. The point you make about the amount of fuel injected is accurate and must be considered when adjusting the AP. For me with my P-38 a center postion worked out well however Based on you findings I will do a little experimenting. Thanks for the info.


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