If you mean a licht clear cracle, Your mixture may be a bit too lean, or compression ratio (CR) may be a bit too high, or a combination of both.
Check the spark plug. It should not be white (lean), or purplish (too high CR)
Just to be safe, add a head shim to reduce the CR, and set the main needle to the point that the plug just is short of turning black. In gas engines, that is the level very slightly rich of best performance. Then lean a tad. (1/32 needle turn).
To check the plug, fly at full throttle a while, then kill the engine without hesitation, so intermediate throttle will not influence the spark plug reading.