Modern Vespa

Just over revs on full revs all the time

Air leak. Check cylinder base gasket, head torque, exhaust clamp, intake manifold for cracks, o rings, reed block and carb clamp.

Thanks I'll try that I've not tourge the nuts so might be that

If, for some reason, you put a new cylinder on hand tight and let the bike rev, you should check to make sure that you didn't break a ring or two before you button it up.