I converted this engine from a 108 trimmer. it is on CH syncrospark ignition and has a Zama 3 needle carb. The third needle is an an air bleed and is blocked off. I have it mounted on a large bi-plane and am running a TF power point 18 X 10 at about 7000 - 7100 rpm.

Recently, at one half throttle and above it has started a "stutter/cough" every 2 minutes or so. It will stutter and then continue running. It does not do this at low throttle settings.

I haven't been able to discern whether it is ignition or fuel related and would appreciate any suggestions or opinions so I will have a starting point to find the problem.

Thanks,

Will

My first guess would be the carb. There may be a spec of something floating around in there or that may be causing a problem on the high speed side. If you had another carb to put on there that would be a quick way to eliminate that part of it.
Its nice if you have another motor or back up parts to try and isolate odd problems like that. Swap carb; swap ignition and try to get the problem to go away. Then you can concetrate on whats causing the problem.
It can get frustrating to say rebuild the carb and still have the same problem, buy a new coil, same problem, on and on.

Dont overlook the obvious; fuel system check for air leaks, spark plug, plug wire, etc.
Adam