morning chaps, i know this is a real open ended question that is super hard to diagnose via the internet, but thought i would try. actually thought i should post on the Bucket thread as there seems to be a few GPs used for racing.
Anyway my issue is i have purchased an ex riding school GP125, removed the throttle governor that was screwed into the throttle tube on the bars to limit it to half throttle.
the bikes seems to go fine in 1st - 2nd gear but occasionally in these gears or more commonly in 4-5 gear it feels like the spark breaks down between 6000-7000rpm, if you go to WOT it just does nothing and dies completley until you roll right off.
My first thought was air filter, so i replaced with a new one, then carb, so i completely disassembled the carb and had it ultrasonic cleaned, this made no difference. i have tried a new plug (currently running a B6 ngk plug as opposed to the B8 heatrange that is factory (but i have tried both). this all made no difference. i cant quite figure out if its fuel or spark, it sure sounds more like spark as it sort of jolts and misses, not your usual rich stutter or lean bog. i put a new coil / lead / cap on that seemed better but still bad. the pipe has been cleaned out and decarbonised, the exhaust port looks nice and clean, it has nice compression and starts first kick when cold.
the problem doesnt become evident until 6000rpm, if you can very quickly "get though" and past 7000rpm it runs fine and pulls like a schoolboy all the way to 10,000rpm, but if you get stuck in that 6-7 range it just wont pull, and opening the throttle wide open it dies completely.
any help would be greatly appreciated, im not new to bikes but not an electrical expert, and this has be rather frustrated.
https://i.imgur.com/6z2KlU6.jpg
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