Did you reoil the engine when you put it back together? The oil help seal the scratch inside the combustion chamber & the engine felt OK with oil. Once you put fuel in, the oil quickly dissolve away! Just a guess!

Take the engine apart & run your finger nail around the cylinder & the piston to see if anything catches. If so, then I am afraid it is going to be a rebuild job. Good luck.

Normanski
Bought a MTA-4 but have No time to run it!