What does the accelerator pump do?

I've heard that the bk mod messes with the accelerator pump, and that the accelerator pump is the reason that we need to stay off the throttle when kick starting, but what does it do. I understand that the bk mod makes the accererator pump a shorter stroke, why is this advantageous?

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2000 WR400F - Throttle stop shortened, Grey Wire Cut, Filter Cover Removed...

The AP shoots a stream of fuel into the throat of the carb for INSTANTANEOUS response. Unfortunately, it can be so INSTANTANEOUS and EXCESSIVE as to flood out the carb, or just make it burble, stumble off the bottom.

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'99 WZ/YR (you choose!) with ALL YZ mods, de-octopused, DSP Doug Henry airbox w/ velocity stack, FMF PowerBomb header, Stroker SX-1 silencer, SS front brake line, OEM YZ tank, IMS YZ seat.

Yeah, its like on older cars before EFI, when you pump the gas peddel a few times before you start it and or pump the pedle when cranking to get it to start. The throttle linkage works a diaphram pump and sends a shot of fuel into the intake even though there is no vacume to pull the fuel from the jets during starting. The pump has no way of "knowing" the bike is not running, so dont hit the throttle when starting unless you know the bike likes it when its cold or you know your running lean on the pilot jets cause of altitude changes or whatever.

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