Compression Test
 

Ok, so I am having a marine survey done on the 242ss I want to buy... as noted in previous threads it has been sitting for 2.5 years... he did the compression test without running it to see the numbers and all cylinders were either 140 or 135 with the exception of cylinder 8 at 125

obviously these numbers should shoot up once the engine is up and running but are these numbers ok for a 1995 Mercruiser 7.4L with VERY low hours on it?

Generaly you want no more then 10% between cyl's when doing a compression test. That one cylinder would have me thinking there maybe something there...A better test would be a leak down to see where you may be getting blow-by. It would give you a better idea if your gonna need to do some engine work in the future because of bad rings...or maybe just get the heads redone (Valve job)...or maybe just a head gasket (But that usually shows up in the compression test as adjacent cyl with lower compression)

It had a leak down test as well that he said passed, I don't really know what the test is or what results to expect so I didn't ask details. The marina mechanic said he has no concerns about the compression test because the motor has not been ran and the numbers should shoot up?? But as we all know, marina mechanics can sometimes be wrong, hence my posting...

what does he mean it passed? Id ask him what the leak down readings were between cyl's
and what psi he used for the test? The cheapo Harbor Freight tools leak down tester is set for like 18psi...which is nothing.

Would definatly need to know what he calls passing or failing. He could generally say that below 30% is passing and be somewhat right but if some cyls are single digits and some are at 30% that variation is very questionable and I don't think of as passing.
Need more info.
In that all are near the top of spec is a good indicator that the mechanicals are not destroyed.

Get the numbers for the leak test . . .

both the compression and the leak test need to be done when the engine has been run for 20-30 minutes. otherwise the results are inconclusive and not really meaningful.