Have you broken it in?
If not, you need to break it in by 'heat cycling' it in your car at first.
Remove the glow plug then using a flywheel wrench unlock the piston from the sleeve.
Replace the plug and start the engine. Raise the idle slightly ('manually') using the throttle. 'Richen' the high end or top end needle by turning it counter clockwise one turn.
Once the tank is empty. move the piston to the BDC (bottom dead center) position. (The flywheel rocks back and forth freely)
Let it cool for a few minutes, then repeat three more times using various throttle positions.
The fifth tank is done driving around a track with you leaning out the top end 1/8th of a turn at a time after the engine is at operating temp.
This should give you a reliable long life engine.