Canalbreath

I've got a 454 mercruiser 420 cyclone Motor started missing the last time out, actually if I disconnect the #5 wire there is no difference so it appears the mis I am hearing may be the engine running properly for a cycle or so, so I pulled the plugs and found #5 almost completely covered in some type of carbon build up. So I changed the plugs and had no difference in performance. I then changed cap, rotor , and wires, with no difference. Made sure timing was right as well. I then decided to do a compression test , all plugs out throttle open and coil unhooked. Good compression in all cylinders.
1-175psi
2-170
3-170+
4-170+
5-170
6-170
7-170
8-170+

Local merc mechanic thinks I tuliped the exhaust valve on that cylinder from my description.
If anyone has any other ideas, thoughts or other things to check or test I would appreciate it. Mechanic wants to start pulling the heads and go to town. I'm looking for any alternative theories.
Thanks All !
~Canalbreath~

magnum350

I had a tuliped exhaust valve on my 350 and had no compression on that cylinder.

Dennis Moore

I would suggest that your cylinders and valves are just fine judging by your compression.
It could be a leaking gasket on the intake manifold.
#5 cylinder (intake port) may be pulling in oil from the valve lifter galley.
Possibly a crack somewhere in the head allowing oil to get in the #5 intake port.
Crack in the intake manifold somewhere.
Intake valve guide leaking/valve stem seal leaking.
Dennis Moore