Tired engines, been there;-)
My dad was in to aeromodeling in the 30's and he bought a few secondhand engines, so when I started I got these really old engines. We would go somewhere with my latest control line plane and spend 4 hours trying to start the engine, by which time everyone wanted to go home. I would then loose/break the plane and build another and it went on... Years later, talking about electro plating he mentioned how he had plated a piston in one of the engine for several days to get it a good enough fit to start reliably!