How does the accelerator pump...

work?? I know all about the BK mod, how to do it, what it does, why it works so well, but what my feeble brain cannot figure out is how does the accelerator pump work in these pumper carbs? So say your stock APJ spray is 2 seconds, and you perform the BK mod and shorten it to your desired lenght. When does this "spray" happen? Only upon initial crack of the throttle? I'm guessing that when the throttle is WFO, there is a continuous spray throught the throat of the carb into the chamber, right or wrong. How does this spray act say just crusing through the gears, 2 second initial spray and then what. Just trying to add to my meager knowledge of carbs, plus I kind of need this info before I do the BK mod. Thanks in advance for educating a dumb a** :)


The pump ONLY pumps from a full closed throttle position to an (half throttle or greater?) open position.

If you are at constant throttle, the AP does nothing.

The BK mod reduces the amount of fuel sprayed into the throat of the carb. Why is this necessary you ask? > The amount of spray is excessive causing an overly rich, flooding condiition.

The APJ makes up for the lack of response that the lower vacuum causes. It gives an immediate surge of fuel into the engine, so engine RPM's & vacuum can increase and draw the fuel into the carb.

Thanks Kevin, Thank's Hayseed.

The jury is still out re:BK on 2002 bikes, even Mr. BK say's the 02's don't need it. So I'm watching this subject very closely. I don't seem to have any low speed bog since going YZ timing, at this point I think I will pass. But it is nice to have a detailed explanation.


I have an 02' Canadian WR 426 with a mean case of the bog when I whack the throttle..I have re jetted several times and if I roll on the throttle its fine...lots of power, no popping on deceleration and smooth midrange power.

I think I am going to go for it.

A more mechanically oriented explanation:

The pump consists, more or less, of the following parts: two springs, an arm, a rod, a diaphragm, pump housing and pump jet.

When you open the throttle the slide pulley releases the first spring which pushes the arm, which is connected to the rod, which depresses the diaphragm, which forces fuel through the pump jet (which is located inside the carb throat just behind the slide). The BK mod acts as a stop on the aforementioned arm, cutting the pump’s operation short. W/out the mod the pump’s capacity is limited by the volume of the housing, once you open the throttle it will deliver fuel only until it runs out. The pump will not refill until the throttle is released.

When you close the throttle the slide pulley rotates the arm and rewinds the first spring, lifting the rod and allowing the second spring, sitting underneath the diaphragm, to push it back up, refilling the pump via a circuit into the fuel bowl (this circuit has a check valve in it permitting fuel to flow in only).

Hope this helps.

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