I have recently bought a boat with a 1978 Mercury Model 110. It takes a few pulls to get it started, and does not want to run in gear for long while idleing. One note is while pulling the cord when starting it, a hissing noise can be heard coming form under the cowling...as if pressure has built up where it should not be. I have looked the motor over and do not see any signs of ripped gaskets or the like. Wold this be a sign of a blown head gasket? I have also heard of a valve (reed valve??) that keeps the fuel/air mixture going in one direction, and that could be failing. Anyone heard of this issue? Suggestions? Take it in for service??