Mikuni pumper problem: XR650R

Greetings all:

XR650R noob (love the bike) here with a question: The 2000 I just bought bogs off idle very badly, as if it were going to die, then it kicks you in the ass! It has a Mikuni pumper carb on it, which I understand is supposed to care of this problem. White Bros E series exhaust, UNI filter (no holes in the airbox) stock engine. Suggestions?


too large of a leak jet could cause too much fuel from the accel. pump and make it bog off idle.

Likewise, too lean a shot, or problem with the accelerator pump not shooting at all will cause the same condition.

And correct me if I am wrong, but I thought a leak jet was designed to control the volume of the shot. Meaning a larger leak jet equates to a smaller shot of fuel into the engine, and conversely a smaller leak jet equates to MORE fuel being forced in..

Too large a shot could well be the issue as I also get a puff of dark smoke from the exhaust pipe when it bogs.

Uh, what's a "leak jet"?

The leak jet works with the accelerator pump. It controls how much "shot" of fuel gets squirted into the carb. Under slow twisting of the throttle-when you don't need a squirt of fuel going into the engine-the accelerator pump pushes the fuel to the leak jet and back into the float bowl of the carb. When you twist the throttle fast the volume of the shot is so great that it pressurizes the circuit and squirts fuel out of the nozzle into the engine. What you need to realize is that the leak jet is constantly "leaking" fuel back into the bowl when the accelerator pump is working. Even when you twist the throttle quickly, there is a portion of the "shot" that is going past the leak jet and back into the float bowl.

I would think that some pumper carbs "may" have a type of leak jet that seals sending 100% of the shot through the nozzle until such a time that the pressure diminishes. Once the pressure diminishes to a pre-set point, the leak jet may open again. This is how it works on automobile carburetors.

Since I have no first hand experience with pumper carbs for motorcycles, this may not be the case-they may have a non-sealable (is that a word?) leak jet-in which case there would always be a portion of the shot being recirculated.

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