Originally Posted by R/C Art

I have two or three thoughts on this problem.

1. bad fuel......try fresh fuel.

2. small particle(s) in the carb.....spin needle out of the carb and flush with fuel.

3. leaky or loose plug.......replace with a new one

I would try #2 first and then #1, followed by #3.

If these three fix attempts are unsuccessful, then I would search for an air leak. First in the fuel tank and lines, then the carb to case junction and then the rear case.

If all of the above fail, then maybe your engine is worn out.