cylinder compression for 86 90 h.p. Johnson

[rbickham]

I have a 86 90 h.p. Johnson outboard. It idles rough and dies at idle. I checked compression and three cylinders have 100 lbs. and one has 70 lbs. I`m guessing the cyl. with 70 lbs. is causing the problem but is 100 lbs. good for the other three?

 

[tashasdaddy]

Re: cylinder compression for 86 90 h.p. Johnson

100 is about what we consider minimum. if one is available check with anohter guage. 70 is horrible. there have been good results using Seafoam Deep Creep, and decarbing the engine, freeing up stuck rings. give it a try before tearing into it. (go to the top of this page, ckick boating forums, then engine faq, scroll to Decarb, will tell you have to do it) the neighbors will hate you as it is quite smokey. then redo the comp test, and give it a short run with new plugs. if still low pull the head, could be just a gasket. if you can get them all at 100, it will still serve you for a while. ost back an let me know how you do. good luck and welcome to iboats.

 

[rickdb1boat]

Re: cylinder compression for 86 90 h.p. Johnson

What does the spark plug tip look like on the one with low compression as compared with the others? Real clean?? Full of fuel?? Carbon? Etc..........

 

[rbickham]

Re: cylinder compression for 86 90 h.p. Johnson

All 4 plugs look fairly wet but all look the same!

 

[Walker]

Re: cylinder compression for 86 90 h.p. Johnson

Go ahead and pull the head on the one with 70. My guess is you have scored cylinder wall in that one and running it will only make it worse. ie more expensive to fix.
If its okay you are only out the cost of a head gasket, Heck, the head gasket could be bad.

 

[Bob_VT]

Re: cylinder compression for 86 90 h.p. Johnson

Pulling the head will not accomplish the de-carb. It will cost you a gasket and a retorque. The purpose of the decarb is to remove the carbon that has built up in the piston ring grooves and sticks the rings.

I just did this in the last few days and I am a believer. Pull your plugs and tip the motor so the cylinder in near verticle. Get that piston up to TDC and let the sea foam sit in there overnight 16-24 hours. Keep checking it and make sure it is filled. At the end of that time drain it out and now recheck your compression. I brought one cylinder up from 70 to 100. It was originally at 60 and I did the decarb while running and that got me to 70.

If that does not work then is the time to pull the head and inspect the cylinder walls.

Overall 100 lbs per cylinder sounds low. Might be time for a rebuild.

 

[Walker]

Re: cylinder compression for 86 90 h.p. Johnson

Cold seafoam won't do a thing. I soaked a piston in pure seafoam for a week and it didn't remove any carbon, didn't even soften it.

 

[emdsapmgr]

Re: cylinder compression for 86 90 h.p. Johnson

A recently overhauled stock 90 should make 125 lbs of compression. The Bombardier Engine Tuner comes in an aerosol can with the direct inject hose. Try that for de-carbonizing your engine. Certainly is worth a try

 

[rbickham]

Re: cylinder compression for 86 90 h.p. Johnson

Thanks, I will try decarbing!!