What's the correct compression on a Johnson '90 120HP V4?

[bleonard]

Trying to bring a '90 Johnson 120HP back to life that's been sitting for 5+ years. Problem appears to be spark, but in trouble shooting to that point, I did check the compression on two of the cylinders. Both read 90-100psi. Seemed low for a 2-stroke, but the shop manual said the actual value didn't matter as much as it being constant between the cylinders. I didn't test the other two once I figured out there was no spark, but reading the forums, people seem to be reporting 100+ psi as normal.

​Any guidance on what the compression should look like? Are readings in the 90 psi range a death knell for the engine?

Thanks!

 

[boobie]

Find out why you have no spark, get it running and do a good decarb on it. Then do another compression test and report back.

 

[jakedaawg]

Squirt some fogging oil in. Let it sit. Crank it over a few times with the pugs out. Get all the xcess out. Compression test all four.

Deoending on why it sat unused it could have been stored with an issue.

Report back numbers. Guages differ. We are first looking for consistancy. Dont use a guage from one of the cheap tool stores that start with H F. Rent one from a parts store.

Start it up and see if we need to go deeper. You can always try a decarb if there is an issue. Many will say sea foam, whatever. The real decarb is a can of brp/omc engine tune and follow the directions to a tee. No such thing as a mechanic in a can.

There is a good post about awakening stored motors in the top secret files sticky at the top of the forum.

Wether or not you have spark should be determined with an open air gap spark tester set to 7/16".

This test should be preformed with all plugs out and after you sent some oil down the holes.

You should also plan on a water pump rebuild after all the testing is done. Dont do it first as all the cranking wont help a new impeller.


This is all just my opinion.

 

[bleonard]

Tested the compression yesterday on all 4 cylinders. 80-90-90-95.

I got the engine to run for ~10 secs on all four cylinders (at least it sounded like it) with some judicious starter fluid application. I probably would have been able to do that on my first try but the kill switch is wired backwards of what I thought and I was killing the spark. Why you would want a kill switch to make a circuit instead of breaking it is beyond me.

 

[boobie]

That's the way OMC is designed. You make a circuit to ground out the ignition.

 

[boobie]

Your compression looks normal on that motor. Still do a good decarb on it using the OMC/BRP Engine Tuner on it.

 

[James R]

I am amazed that people still try to run what might be a perfectly sound motor without rebuilding the carbs. If you dont have a good fuel supply you dont get lubrication but you do get a blown motor.

 

[bleonard]

Carb rebuild is next on the list. Spark plugs were soaked and I can smell the fresh oil in the mixture in the carb. I wouldn't subject the poor thing to running dry!

 

[bleonard]

Rebuilt carb - disconnected oil on VRO2, switched to pre-mix and motor is running and happy! Mechanic buddy suggested that I check the compression after running for a bit. Said that the piston seals might seat a bit better after that. I'll see what the compression like in a few weeks.