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I upgraded my scooter to a motofine 72cc BBK. It was an easy install and it ran great for about 70miles. Then it started to bog down when i try to get past 20 mph. If i keep gasing it it will get past the bogging some times, but it takes a lot of work to get it there. I had a 85 jet in it, so i tried putting in a 90 and there was no different. I have the stock air box still on it.
Could I have lots some compression? I am using the head that came with the kit. Should i try putting the stock head back on?
I am grateful for any help.

Hello, I know this topic is old, but I came to the same issue. Upgraded to BBK 72cc and changed the main jet to #85. It ran fine for 500km, but then, suddently, the power was gone and it ran very slowly. My serviceman found that the carburetor had bad seal (sucking more air) AND the plug was black. This is a contradiction to me. Anyway, the machine runs much better now but still not as good as before. It is unclear what had happened.