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Yz125 surging issues

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99 yz125. Fresh top end (or so previous owner says) friend of a friend. Runs great bottom and top, but mid surges. Like rev, bog rev bog. Messed with mixture screw to almost no affect. Also loads up if you don't rev it out, loads up quickly. What would cause that?

Edited by Joel Tilford

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The mixture screw isn't going to help the midrange much at all. That's mostly the needle, and the needle clip position.

I have played with the needle position, it did not help. Dropped it 1 and raised It one to no change at all.

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I read a post recently describing a similar condition on the same bike. The OP said he had luck with cleaning every port of the carb, and specifically the two small passages at the bottom of the carb inlet mouth. There is a small mesh screen and a hole. Clean these out good and it should restore power. Can't remember what they are called but they work the 1/4 to 3/4 range of throttle.

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