Question about compression readings changing.

[sutor623]

Ok guys I have a sweet '84 evinrude 75hp. Runs great. Nicest outboard I've ever had actually. Anyways motor ran great when I bought it, compression read 138-140psi across the board. Me not knowing exact history did a full fuel and water system overhaul. Even put a new fuel pump in it. So I did a link and sync and motor is running perfectly. Put 10 hours on the motor in the past few weeks and decided to pull spark plugs to check for lean conditions via spark plug diagnosis, and decided since plugs were out to redo compression test. The gauge I have has been around for about three years and I have tested many motors with it. From 0psi to 220psi in a car. Retested compression and it read 150 psi all across the board!! So what gives, why the change? Obviously I used the same gauge both times.

 

[bigpoppakdog]

Re: Question about compression readings changing.

did you do the compression test the first time with a hot motor and the second time with a cold motor?

 

[boobie]

Re: Question about compression readings changing.

Sounds like your rings are good and free and everything is working properly.

 

[sutor623]

Re: Question about compression readings changing.

did you do the compression test the first time with a hot motor and the second time with a cold motor?

Both tests were performed with a cold motor.

Sounds like your rings are good and free and everything is working properly.

Do you think that the rings were a bit stuck and by running the motor it freed them up?

 

[boobie]

Re: Question about compression readings changing.

Yes................By the way, how's your lower unit doing ??

 

[GA_Boater]

Re: Question about compression readings changing.

I vote for running the motor has freed the rings. Any idea how long the motor sat before you started using it? If it was a long time, the oil thickens and causes the rings to stick some. So you have fresh oil and the rings are nice and free.

 

[sutor623]

Re: Question about compression readings changing.

Yes................By the way, how's your lower unit doing ??

lower unit is doin great, thanks! I scratched the three blade impeller and she is cooling nicely.

I vote for running the motor has freed the rings. Any idea how long the motor sat before you started using it? If it was a long time, the oil thickens and causes the rings to stick some. So you have fresh oil and the rings are nice and free.

Thanks for your input GA, as always. The motor had sat for months to a year when I got it. Also the motor was fogged wen I checked compression, I hadn't run the fogging oil out yet. Didn't even think about that til just now.... But boy am I glad that I bought her! I have never tested a 2 stroke outboard with this gauge and gotten numbers this high.