I've been banging my head against a brick wall on an ATC200E for the past couple of weeks. The danged thing will start and run, but looses power at a little less than 1/2 throttle. One thing I've noticed is a lot of blowback through the carb when it starts loosing power. Compression test shows 150psi, timing marks on cam/flywheel are correct, valves are adjusted to .003", and timing light shows proper advance. Carb has been rebuilt with a Keyster kit, Intake valve has been replaced, and seating verified before putting the head back on. Muffler has been removed to ensure no exhaust blockage.

Anybody have any other good ideas?