4.3 Merc listed the compression pressure 180 psi in late 80 & 90's service manuals. Late 90's and 2001 manuals, they Merc listed it as a min of 100 psi. Anyways for whatever its worth.
Also a leak down test is not 100% full proof. One example, I have seen some that shown around 8 to 12 % leakage and a compression test of 65 psi. Broken valve spring and when you put compressed air in the cylinder it helped push the valve tighter against the valve seat to help seal it. IMO both are important but not 100 % full proof.
The 4.3 / 262 CID is still not as relieable as a 5.7L 350. We see some valve problems from them and the carb intake manifold has it own drain plug to drain water out for winterization. Many people and shops miss that drain plug on the carb intake as it easily freeze breaks. Also very weak lifter valley more so on the port side will crack very easy from improper winterization.
The V-6 marine engines hate to be over reved if you do not have a rev limiter incorporated in the ignition system.
IMO I would not rule out a vacuum test for testing the internals of any motor. Marine MPI engines can have a tad lower vacuum reading of 15 hg or 16 hg even from a brand new motor - the reason is because of the IAC system incorporated in the intake. The old norm for a good motor at idle was 17 - 21 hg for a vacuum test and basically is the still norm for carb motors. .
Motor you are correct about the specs of 180 psi but as I said Merc changed it to a min of 100 psi for the current 4.3 's in their listings of the service manuals. the thing about compression testing is to have close to even #:s across the board and IMO 140 and up would be fine for a V-6. If I had to pick 2 test only, I probably would do a vacuum test or a leak down and a full blown lake test.
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