It will be difficult to get running right with a 12mm carb, a 70cc cylinder requires 40% more air/fuel mix per stroke which I suspect it's struggling to deliver at higher revs.
Preferably a Delorto 17.5 or maybe a 19mm , there's no real gain with a 19mm and you start losing throttle response if you go any bigger. Pedparts recomend a 17.5 and that's what I run with a stage 6 streetrace.
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