My GC135 engine had the exhaust valve head break off. I torn engine down and the only damage appeared to be a few dings in the piston head so I replaced the exhaust valve and reassembled the engine. Now it won't start. It tries to but seems as though it doesn't have enough compression to actually keep runing. I removed the plug and attached a air line in the plug hole and I feel and hear air coming out from the exhaust port when the valve is closed. Just wondering if anyone out there has ever had this experience or knows how much of a compression leak you could get by with and still have the engine run.