My 489 with 15 hrs on it has had low compression (100psi) on #4 since the eng builder fixed the blown head gasket. All other cyl are 170 psi. After complaining to the builder, he said I need to take the boat out and "blow it out/put some hrs on it" and it will work itself out (at the time only 5 hrs on it). Pretty skeptical about this, but I wanted to use the boat and got tired of messing with it, so I put 10 hrs on it and the compression made it down to about 60 psi on #4. Leak down test revealed air leaking through the intake valve. I verified 0 lash. Then the other day, I fired it up, sounded unbelieveable, power is back, compression is 170 psi #4. What could have caused this??

i would say carbon but its basicly new..may be a slight band in the valve..i would still keep a eye on it..look close at the valve spring on the intake and make sure its not broke,could be stacked now and keeping the valve sealed..