Compression testing and rebuilding

[mav6759]

I have an 1988 evinrude 60hp, I did a compression check on the motor and the numbers were 110, 90, 60, I trying to fine out what these numbers mean. I want to rebuild the motor, but I was told that you can tell if there was a bad cyclinder or a bad head gasket by the numbers of the compression check. What is meant by low compression, what causes low compression and what should I look for when rebuilding this unit, that may have caused the low compression.

Mav6759

 

[Benny1963]

Re: Compression testing and rebuilding

two of those numbers are low bad you need to pull heads and see what kind of damage you have. cufiing due to overheat /lean conditin from carb /improper timing
you will need factory manual to go very far
good luck bennyb

 

[Lakester]

Re: Compression testing and rebuilding

What is meant by low compression, what causes low compression and what should I look for when rebuilding this unit, that may have caused the low compression.
Mav6759

hello mav,

simply stated:

low compression means that when the piston comes up in the cylinder and compresses the air/fuel mixture it is not sqeezing it enough. usually due to a mechanical leak such as bad rings, head gasket or piston to bore integrity.

low compression is usually caused by exceeding useful serivce life, worn out parts, or broken parts or related matters.

you should look for grey to black discoloration around the inner ring area of the head gasket indicating [leak] blow-by. the head bolts can loose their set over time and release their clamp grip on the head gasket. you should inspect the rings to determine if any are broken. you should inspect the cylinder walls for aluminum scuffing and/or deep grooves. improper oil to gasoline ration could also cause this.

you can learn a lot from the factory service manual for your particular year and engine size.

good luck!

regards,
lakester

 

[iwombat]

Re: Compression testing and rebuilding

That said, you may not have to rebuild this motor. Have you done a decarb? It could just be a case of stuck rings due to carbon build-up.

 

[thefairlaneman]

Re: Compression testing and rebuilding

Im not an engine expert but I can say with little doubt that the one with 60lbs most likely indicates a serious issue. Mine had 60 in one cylinder and I had to remove the head to see the problem. Badly worn wrist pin had caused the piston to wear almost ALL THE WAY THROUGH the cylinder wall. This cant be fixed. = new power head or complete engine.

 

[tashasdaddy]

Re: Compression testing and rebuilding

if they were both 90, i would suggest a decarb, but the 60 says pull the head.

 

[iwombat]

Re: Compression testing and rebuilding

Yeah, you're probably right. 60 is kinda out there. Although we should also ask about the technique for reading compression. When I see numbers continually go down like that they're a bit suspect.

Did you take another set of readings starting at the low cylinder and work back to the high? Did the readings match?

 

[HighTrim]

Re: Compression testing and rebuilding

Also do the compression numbers decrease, from top to bottom?