Neel,

You may need to check the fit of the piston and cylinder liner. If it's worn, then the engine will tend to not run well once it's warmed up to operating temperatures.

An ABC or ABC-type of engine should have a bit of tightness when the engine's cold and the pistion is moved to Top Dead Center. This would be noted with the glowplug removed.

If the engine does not have that tightness, then the fit is too loose.

If your engine is the O.S. Max .61 FSR, piston/cylinder assemblies are still available. In the U.S., the part is:

OSMG4934 26703200 CYL & PISTON 61SFN US$114.99

The piston/cylinder assembly for the .61 FSR and the .61 SFN are the same.