compression test

[fish man]

i'm going to be a first time buyer would like to do a compression test on a 95 115 merc ob please explain how and what result should be thanks

 

[JB]

Re: compression test

Howdy Fish man. Get a screw-in type compression guage. Have the seller crank the engine and warm it up. Remove all spark plugs. Screw the guage into each spark plug hole, crank the engine over four or five turns. Record the reading on the guage and move on to the next cylinder.<br /><br />Each cylinder should compress 100psi or better, with no more than 10psi difference from top to bottom.<br /><br />Good luck.<br /><br />Red sky at night. . .<br />JB

 

[fish man]

Re: compression test

thanks j.p. for the info. are there any other things i should check before or after sea trial?

 

[JB]

Re: compression test

Yes, Fish Man. Examine a sample of oil from the gearcase. Loosen the bottom screw-in plug. Pull it out quickly and then replace it after wiping it on your finger.<br /><br />Rub your fingers together and feel how much metal fragments are there. They should be minimum or none. The oil should not be milky and no water should have come out when you pulled the plug.<br /><br />When you have the spark plugs out for the compression check examine them. They should be grey to tan, dry and without a lot of deposits.<br /><br />On the sea trial: The engine should reach 5000 to 5500 rpm at full throttle, but not over 6000. (Confirms proper prop.)<br /><br />That's all I can think of.<br /><br />Red sky at night. . .<br />JB