1986 50 hp compression.

[wvmedic]

I have a 1986 50 hp. 3 cyl. Mercury was checking the compression and all three cylinders were at 140 psi. Is this good compression for this particular motor? I know for a car or a motorcycle it would be on the topside of good. But for boats I?m on foreign ground.

Also while checking the compression I checked for spark, shows good on all three cylinders. Fuel filter is good and clean. So hopefully when my lower unit gets here, should be tomorrow the motor should fire right up correct?

I added the other stuff so if I have over looked any thin someone might give me a heads up?

Thanks Jeff.

 

[JB]

Re: 1986 50 hp compression.

Don't get preoccupied with psi, Jeff. As long as it is above about 100psi, that is fine. In some cases as little as 70-80psi is okay if all cylinders still are a close match.

What is important is the span. It needs to be within 10%, and yours certainly is.

The real purpose of compression checks is to look for one or two cylinders that may be well below the others. That would indicate trouble. The probability that all three would go bad the same amount at the same time is about zero.

 

[emckelvy]

Re: 1986 50 hp compression.

Jeff, more important than the ultimate value is the variance between cyl's. If you're getting even readings across-the-board on all 3, then that sounds pretty good. On most of the Merc's I'd start worrying if compression readings vary more than 10 psi between cyl's, and have seen plenty of cases where even a 5 psi diff was pointing out an upcoming problem.

More accurate than a compression check is a leak-down test, but that requires other test equipment.

http://en.wikipedia.org/wiki/Leak-down_tester

For the average boater, tracking compression readings from season-to-season will give you a good assessment of internal condition and will allow you to spot a deteriorating trend.

Once you get it back together, treat your motor to a Seafoam decarboning treatment and that'll keep the rings very happy indeed!

Check this site's FAQ or here for Seafoam info:

http://www.seafoamsales.com/motorTuneUp.htm

I like Seafoam Deep Creep which is just an aerosol can of Seafoam, easier to spray into the throats of the carbs:

http://www.seafoamsales.com/deepCreep.htm

HTH & Happy Boating..........ed

 

[wvmedic]

Re: 1986 50 hp compression.

Thanks JB and emckelvy, I do plan to seafoam it both spray and fuel tank mix. I use seafoam on all of my autos.

Jeff.