RE: questions and worries
Hmm all I can think about is you are still running very rich and that is the fuel you are seeing come out after the engine shuts off. Have you ever looked at a car exhaust or motorcycle exhaust. There is black inside all of them after running them. I think the same thing happens to rc exhaust. I dont think it should be black gunky stuff though. If the motor is running well I would worry. Also pinching the fuel line will cause the engine to rev higher, because you are cutting off the fuel to the motor. Kinda like leaning the carburator. It cuts nitro to motor causing the motor to perform better but increase in temperature. It will eventually kill the motor you probably just didnt hold it long enough. Or didnt pinch it hard enough. Also plugging the exhaust will flood the motor thats why the nitro came out the exhaust. Best way to do it is pinch the fuel line or stop the flywheel from spinning. But I always plug the exhaust because its easier and doesnt cause any harm.