I think my $18 GS 21B01 is finally giving in. It seems to have good compression and runs great with 1-2 pulls to start up when cold with a solid idle (can sit there forever without needing to blip throttle), but after a tank of running the car on the track or behind my backyard in the mountains the engine becomes easier to stall with a weaker idle and when it dies it gets difficult to restart. The compresison feels milder and when I plug the exhaust to prime the motor, it doesn't suck the fuel through and I'd have to blow through the preasure hose to push the fuel into the carb. Also when I disassembed this motor the night I received it, I noticed it has 2 head shims (not sure how thick they are) and I run 20% nitro/16% oil- would it be safe to take one shim off to see how it responds? I've replaced fuel lines, glow plug (McCoy #8 & #9), used a freshly charged glow starter, cleaned air filter.

Any ideas? I guess this is a sign of needing a rebuild, I knew $18 wouldn't last long.