If the head gets tightened-down evenly and still has problems, let me know. You may need something as simple as a new gasket, or you may need a new head...the original one could have become warped.

It's normal to have some leakage past the piston ring when you turn the engine over slowly. You should hear some hissing.

With a fast flip of the propeller, though, you should only hear a "pop" as the exhaust port opens after each brisk flip.