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01 YZ426 Carb Issue

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Ran a hare scrambles in Texas about 3 weeks ago. Bike ran great. Returned to CO, rejetted to specs I've been using for 4 years and bike has developed an odd problem. Any help would be appreciated.. here's the tale:

As the bike builds in revs off the bottom end it develops a stumble that continues until revs build and the carb circuit appears to hit the upper end of the slide and main jet, where it runs cleanly. The stumble acts like a fouled plug or like it's hitting on every other firing of the plug.

Plug has been changed.

Carb has been completely disassembled and cleaned.

Replaced carb with a buddies carb and the bike runs great.

Changed my jetting (was already +/- 1 size in all circuits) to match his.

TPS ohms out to spec.

Gas has been changed.

Choke circuit cleaned

DRD hot start replaced with stock unit.

Changed slide/needle with unit from good carb.

Valves checked and well within spec.

Nothing fixes the problem.

The only thing I can get to make a difference it to change the needle to full lean condition. Then I have a flat spot about 1-2000 RPM's higher in the power band.

I'm stumped. Any ideas, suggestions?

Merf

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