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Compression Check

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Ray Mac

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" I have twin 454 Mercruiser M

" I have twin 454 Mercruiser Mags. Port motor seems to have a uneveness to it at about 2000rpms .Cant tell if it is a miss .I did a compression check,all cylinders were about 135 to 145 except for #6 it read 105.Is this within acceptable range and be the problem ?What would be my next step? Thank you "
 
"That's borderline, Ray.

"That's borderline, Ray. Inspect the plug from #6, looking for deposit or discoloration when compared to the other plugs. I'd do another compression check on all the cylinders with the engine warmed up and see if the numbers are nearly the same. If so, I'd then do a "wet" compression check on #6 by squirting some oil in the cylinder and repeating the test. If the compression improves noticeably, the rings have blow-by. If not, valve(s) are not sealing properly.

Lastly, if the numbers are still bad. Do a leak-down test on #6 to confirm loss of pressure. Just Google on "leak down test" if you need a description.


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