TOPIC: 1980 90 hp Mercury Comp Check

I have done a compression check on the motor using a socket and pull handle. after about 5-6 slow turns It has 90-95 on all 6 cylinders. I cant turn it very good like the starter would. Is this not an accurate way to check it?? Thanks
Micheal

you need a good tester and you need to get the motor cranking with the starter. 125 lbs range even on all 6 cylinders is what you are looking for. A difference plus or minus within 3 to 5 lbs on the cylinders is tolerable. If you start getting below 120 she is getting mushy.

g3jim wrote:

you need a good tester and you need to get the motor cranking with the starter. 125 lbs range even on all 6 cylinders is what you are looking for. A difference plus or minus within 3 to 5 lbs on the cylinders is tolerable. If you start getting below 120 she is getting mushy.

Depends on the compression gauge...some will read higher\lower than others.

Your looking for consistency across all the holes more than high numbers. Ive seen motors with sub 100 psi numbers across the board that ran like an absolute rocket ship...and ive seen ones that were 150+ that just ran ok. As long as all the holes are within 5ish pounds of each other, your good to go. If there is more of a variance, it could mean there is a potential problem or something is on its way out(bad headgasket, bad rings, scored cylinders, etc). Some gauges also dont seal as well as others...ive got one that reads a consistent 10-15 pounds lower on every motor i test it on than another gauge i have. Im more worried about even numbers than high numbers.

Cranking with the starter is a much more accurate way of determining compression. A little bit of oil down each cylinder will help ensure correct readings if the motor hasnt been ran in a while. Install your gauge and crank 5-7 complete revolutions or till the gauge stops climbing.

Slow afternoon stuck indoors, so here's another two cents worth:

If you're getting 90psi cranking it by hand, I'd say you're probably in good shape. You really need to spin it with the starter to get a good reading though. (Just curious, why are we not using the starter to do the check?) It's true that different gauges will give different readings on the same motor, but as stated you're looking for consistency as much as minimum pressure.

Also, you may want to use the search function (upper right hand corner of the screen) to look up "compression test (or compression check)" and see what has been written here about grounding out or disabling the ignition system while performing the test.

While I understand the thought of lubricating the cylinders before doing the test, I'm told part of a thorough testing procedure is doing it without oil first and record the readings. Then squirt some oil in the plug holes and do it again, if the readings come up significantly it indicates worn rings.

you can also do a leak down check to be sure of the health of the cylinders. just a bit more time consuming is all.

The engine has sat since 2006 (last registration on donor boat) The starter solenoid is missing. The engine had some wiring issues and shorted out (melted) the wires and solenoid. The wire shielding had turned to dust and got together. The friend I bought it from bought it from an county auction. While re-wiring the burnt and bad wires he mounted the motor on a pontoon boat.. Well he quikly found out it was to tall to clear the rails of the boat... So he stopped on it and got a Johnson 85hp that would clear under the rail when tilted. I have the engine on a stand while the Caddo is in the paint shop. And I need to buy a solenoid to replace the melted one.....

Thanks for all the GREAT help.

Micheal