Hi I have done som more running on my engine and did change to NGK cm6. No change in behaivior with the NGK plug.But more running ca 1liter gasoline, mede it run better. On some point on the rpm curve the engine vil be eratic. It look like this is a carburettor problem but it may be bad conection on the ignition due to vibration. Looks like this engine will work better with more running time.