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XR 250 idling high

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Just purchased a 2000 XR250 with 33 Mikuni pumper carb, white brothers exhaust and 280 kit. Bike runs strong but doesn't want to low idle. Really makes it a pain to start. I don't know anything about pumper carbs. I had this problem with another XR years ago and it was because of too much back pressure. I don't think that is the case here. Any suggestions would greatly be appreciated.

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I believe the Mikuni has a fuel screw like the stock carburetor. It fine adjusts the pilot circuit mixture. Turning it clockwise leans idle mixture and counterclockwise richens it. Play around with this adjustment. Next step would be to change the pilot jet (richer?)

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I would agree with looking at the pilot system. A slow (or never) return to idle is an indication of a lean pilot system. You can adjust the screw out to lean. If you get more than 3 turns out, then you need to go up one size on the pilot jet.

When the bike is nicely warmed up, rev it up and cut the throttle. If the pilot is lean it will hang up in RPM before finally returning to a nice idle. If it is extremely lean, it will never return to idle. Richen it until it has a nice, quick return to idle. That is how I dial in the pilot system.

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sounds like its running lean. Also, what cam do you have. if its a 280 motor possibly it has a racing cam in it. That also could be a reason why it's hard to get it idle low. I would also check for intake leaks by spraying some carb cleaner around the intake.

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Thanks for the advice. I may be dumber than I thought because I can't find the mixture adjustment screw. All I see is the choke and the idle adjust. I started the bike up again tonight and if I turn the idle up it will run fine. If I turn the idle down it will run for about ten seconds or so then die. I can't check the plug because I don't have a tool for it. The spark plug socket I have for my old 87 XR250 is too big to get down in the hole. I will have to oreder that tomorrow. As for the jetting I get I will need to research a little and find out what jets I should have and pull my carb and compare.

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I can't find the mixture adjustment screw

It is directly below the bore of the carburetor on the engine side. That is, it points directly down and is located on the round part of the carb that is clamped into the rubber boot on the engine head. You need a very, very short screwdriver to adjust it. I cut off the end of a small screwdriver to make a special tool.

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Hi, Make sure that there is no air leaking into the inlet tract, from places like the join between the carb and the manifold, etc. This will cause leanness, which is un-tuneable. I hope this helps.

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