Re: 66cc won't fire?
A bad CDI can cause your problem, they burn out fairly easily. Remove the plug and with it connected to the spark plug wire hold the plug close to the motor and pedal and see if you have a spark. If there is a spark then it's good to go and there is an issue somewhere else. The other issue could be a restriction in air flow in the exhaust.
I literally just had this same problem and did the following:
Replaced plug with a new one. No luck
Replaced CDI. No luck
Cleaned air filter and checked carb. No luck
Took off banana pipe exhaust and flushed with straight gasoline and reattached. The bike started on the second pedal.
Currently no bike but thinking about starting a new build soon.