I have an 80cc grubee that i just replaced the piston on and it will not stay running under its own power. If i pedal it for a while it turns over, even gets up to running temperature but will die as soon as i stop pedalilng. Stuck.
New rings as well? I had a similar problem a while back. Turned out the rings were defective: they had a slight inward bevel that allowed expanding gas to pass over them. A used pair I had laying around completely resolved that heartache.