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Carb Guru Input for Various Questions

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Hi guys,

Long-time lurker, first time poster here. After having browsed thoroughly trough the archives for info on jetting, I figured I would query the experts here on a few issues that have been nagging me.

Some background info: I'm currently trying to get my xv500 jetted correctly. I'm using a Mikuni vm44 carb with no accelerator pump. So far the bike will idle really well, but from what I can tell, the needle jet and the main are too rich. Anyway, with that in mind, here are my questions:

1) If I quickly whack open the throttle from idle to about a 1/4-1/2, the bike will stall out. If I understand correctly, this isn't actually a "problem", it's just what happens when you don't have an accelerator pump carb. Ergo, it isn't something you can do much about, save for not opening the throttle that wide, that fast. Correct?

2) After a hard deceleration, or quickly letting off the throttle from 4000rpms to idle, the bike will lose a lot of rpms, bog, and almost stall out. If you touch the throttle, it will stall. If you leave it alone, it maybe will stall, or just slowly work itself out and come back to normal 1300rpm idle. From what I understand, this too is due to a mechanical type carb, with no dashpot. When the slide is quickly whacked closed, this creates a super rich mixture that then gets sucked into the motor, bogging it out. A dashpot doesn't allow the slides to close as quickly, therefore preventing this type of stall. Correct?

3) (This is a little different question). Apparently, a vm44 is a fairly large carb for a 500cc motor, in that it will flow a lot of air. I have heard that maybe I should have used a 36mm instead. Will installing a restrictor plate accomplish the same thing as "going down in carb size", in that the big carb will now have the same flow rate as the smaller carb?

Anyway, I'll leave it at this for now, as I don't want to write up a dissertation. My biggest issue is finding out if the above queries are "correct", in that I can't do anything about them, or if in fact the bike shouldn't be having issues like that at all, and therefore my jetting is waaaaaaaay off.

Thanks very much guys -Hansol

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1) correct.with out an accelerator pump it will never respond to abrupt throttle opening from idle. many people make the jetting way rich to compensate and that just make sthe bike run bad over all.

2) no you have a float level and or pilot circuit setting issue.sounds liek the pilot circuit is way rich.

3) you are one vm44 on a 500 twin? so you actually have a 44mm on a 250 single cylinder (only cylinder firing at a time).so yes its way too big.

a 36mm vm shoudl be easy to find.

i highly suggest going to a cv type carb.

3)

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Eddie,

Thanks for the input. I went out and snagged a VM36 (as suggested) to test out, and it seems to have solved most of the problems, save for the "bog on quick opening" - but you already called that.

Anyway thanks for the answers. It certainly put me on the right track.

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