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yz85 strange mid range upper rpm bog / flutter

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https://www.facebook.com/kenkreger1/videos/10155094244070903/

See vid in link.

Put a new piston in this bike, bore was .001 Ran good for 2 tanks of gas now has this weird flat spot surging in mid to upper power band. Even with no load you can see it just does not want to rev. Cleaned fuel petcock and carb, new plug. Changed the reed because the stock ones were open about 1 mm. Has Boyeson now. Compression in 135. Good fuel flow out carb drain. Starts good, runs normal on the bottom end of the power band. Pulls good as long as you don't rev it too high. New air filter. Has no power valve on this model. Kinda stumped. Ideas?

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Pulled the pipe/silencer and checked, no blockage, steamed it outwith the hot pressure washer, but no love there either. Wish I had a spare carb to try

Still al oem in the carb, work great last summer

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My son's was doing the same thing,I dropped the needle clip 1 step and all is well,just an idea from my experience.

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Next we are going to do a few runs in the power band where it is pulling correctly and kill it quick to check plug color. it did look a bit lean last summer with a br9 (1 hotter plug)

We tested out the the wiring the other night and all coils were with resistance ranges listed in the manual. The only other thing that could be wrong in the spark realm is the cdi which cant be tested. Does not sound like a spark cut out to me. It will easily jump a .5" gap on the tester without misfire, 2x what the manual says.  Pretty much down to air leaks into the crankcase or something that change in carb. I'll try richer or leaner on the carb next. Failing that, I'm going to pressure up the case and look for leaks.

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