70 Lbs & 90 Lbs Compression ... Run it or Rebuild it ?

[bobgritz]

I picked up a 1988 Johnson 15 HP outboard and it runs okay but is showing 90 pounds and 70 pounds of compression. I assume they should both be within 5% of each other and also should be over 100 lbs. Any thoughts if I am okay the way it is or could I do damage due to the 20 pound discrepancy?

 

[sutor623]

Did you know there was a compression issue when you bought it? Is this a pull start model? The pull starts will normally register 70+ pounds due to not being able to get the speed/torque out of a starter. My 1980 9.9 was a pull start and had 75# in each cylinder and ran perfectly.

All that being said, I assume you are well versed in compression testing? To be extra sure you are getting accurate measures you can put some teflon tape around the threads of the compression tester and make sure that you arent losing compression from the spark plug threads. Next thing I would do is pull the exhaust housing cover and peek in through the exhaust ports to look for cylinder scarring. After that checks out, pull the head, give her a good look over for cylinder scarring. If all looks well, you likely have a cylinder head gasket problem. If thats the case, slap a new gasket on her an let her fly!!!!

Is the cylinder that is showing low compression the upper or lower cylinder?

 

[sutor623]

One last thing. If this motor has been run in salt water, likely you will snap off some bolt heads after removing them. If you are willing to pull the powerhead, and hit the drill press, then go for it. Another thing that you can do is hit each bolt with heat, spray PB after it cools some, let them soak overnight, and then try them the next day. Lots of times a 3/8 impact wrench will get the bolts out without snapping them. Be careful and good luck!!!!!!!!!!

 

[bobgritz]

Thank you. I should have mentioned that I have already pulled the head and thermostat cover. The cyliinders looked unblemished but I have not pulled the exhaust cover to inspect for scoring on the pistons. I replaced the head gasket, the thermostat cover gasket and installed a new thermostat. Wow, you are right on regarding salt water and the difficulties encountered when removing bolts. I am a huge fan of 'heat' and without it salt would usually win. BTW, the top cylinder has the 90. I never realized that a pull start test could deliver a lower reading than a starter motor compression test. Interesting ... thanks ... I'll add that to my brain. My test was performed several times with the same results (pull start) so it is perhaps safe to assume that there is a 20 lb differenterial (whether a manual or an electric starter is employed). Given the fact that you were successful running an outboard with 75/75 leads me to believe I should be okay, at least until I decide to do a complete tear down!

 

[boobie]

Maybe do a good decarb on it ??

 

[Crosbyman]

engine is 27 years old...maybe it is not worth the trouble and expense if it runs...just run it


remember.... don't fix it if it works ! just try Boobie's suggestion

 

[ttankmoran]

I would still want to pull the bypass covers and take a look at the ring packs, there is certainly a good reason you are 20 lbs down on a cylinder. If its just stuck rings a good decarb could do a lot of good. These are good enough running and quality engines where I would spend the money to get one up to snuff.

I have a 1975 9.9 and its still one of the best running small engines I've used. Gauges differ, but just as a comparison my cylinders register right at 100 psi. Fresh rebuilds will generally be in the 110-115 range.