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Accelerator Pumps

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I have never tried it, but I now people who did and all they got was increased flooding.  The carburetor they used was similar to a Mikuni with a pump on it that I use on one of my xr's.  They had the rig running well on the 2 stroke with the pump deactivated,  just to make sure it would even work.  That step required a thinner needle and the slide wasn't right.  They tried some leaner needle profiles to try to make the pump work and not stumble several times.  The thing would do the opposite of what they wanted and did what was described as a "delayed" fall on it's face.  I have thought about what made that happen and figure that the two stroke has more trouble atomizing fuel to begin with and that the amount pumped into that one was probably too great.  In the 4 stroke the path is shorter that the fuel is going to take in the two stroke,  and less time under greater vacuum (part of what some call "fuel pulse") so you get a chance at better evaporation from the increased atomization without the chance of puddling.    The bike was a KDX 200.  The factories started sending carburetors with an electrically controlled system to richen up the mix at large throttle openings at moderate rpm ranges and then lean it back out as the rpms rose,  so that indicates the need was recognized.  This started happening more than 15 years ago,  and up until then there were few dirt bikes with a "squirt" circuit either.    The pump on the two stroke that I mention, though no where near tuned for it,  made it a cinch to flood!!  

The industry probably decided that a 2 stroke that was going to smoke more wasn't a good idea (smart for the times) and one that probably would have gas mileage suffer some was not going to make it on the street with the smoke etc thrown in,  so no one tried it at the manufacturers level.

FYI the guys that tried it have built some pretty decent pipes for 2 strokes and 4 strokes and if any renegade builders could figure it out they probably could. Typical of two people who work with eachother well,  and one thinks it will work,  and they other is trying to see if his idea that it is a waste of time is the better path :ride:

Anyway,  if I ever seriously thought about making one work,  they convinced me (without trying)  it wouldn't be worth my time.   But I wondered from the 60's why they were on cars,  and at the time were not on  ANY motorcycles.

 

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Because they don't need them.

Two strokes provide the same relative vacuum across the jets at low rpm and up.

Four strokes do not. There is a huge void of vacuum at lower rpms, that needs to be dealt with, using an additional squirt of fuel.

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Because they don't need them.
Two strokes provide the same relative vacuum across the jets at low rpm and up.
Four strokes do not. There is a huge void of vacuum at lower rpms, that needs to be dealt with, using an additional squirt of fuel.

Thx Krannie,
Makes sense...can you say more...if you know? Why do 2-strokes provide similar vacuum at low and high rpm and 4-strokes don't?

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There is only draw of vacuum across the jets for one of the strokes, while with two strokes, it's on every up stroke.

So, for three strokes, no vacuum is occuring across the jets, so they draw nothing, and then all at once a need for fuel arrives, and it can be delivered soon enough or fast enough, so the pump is there to fill in the gap. At higher rpms the apump still works, but at has nearly no effect.

Ride an XR650R without an apump, and one with, and you will never forget.  With one you can try and go over an obstacle, and with the other it's easy.

Edited by Krannie McKranface

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