50 when i first start it up at wide open throttle runs ok for 2 mins then start bogging down then picks up speed then bogs down again will not speed up after that will accelerate up and down. Replaced impeller, tstat, plugs. Cylinders each have 150 compression. It was overheating due to blockage was able to clean out and fixed that problem, powerpack was bad so replaced it. Could it be a blown head gasket, off timing. Please help.
