Idle Speed Screw

I have a 2004 YZ250F and just recently I am having trouble when I up the Idle Speed with it backfiring badly. It will not adjust up as high as it did last year when I bought it. I have a brand new plug in it. Any suggestions?

Is it truly backfiring, as in flame through the airbox? Or is it banging/popping through the exhaust?

A true backfire would be due to a rich condition or preignition. While popping through the exhaust is typically a sign of lean jetting. If the popping occurs on decel it is likley the pilot circuit is too lean. Try adjusting you fuel screw and/or going to a larger pilot jet.

Rick

Something that occasionally happens to the '03/'04 models is that the plastic nut that anchors the hot start cable breaks, sometimes allowing the plunger to back up part way out of the carb and be partially open all the time. This creates an exremely lean condition that the engine doesn't like at all. Pull the tank and take a look (sometimes you can see it well enough with the tank in place).

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