Johnson compression #s

[putback]

I gotta ask, can’t find it. What should the compression be on a 1962 Sea horse 40 hp RK24L. I have 90 psi on both cyl. Can’t find any where what they were new?

[wiscoboater]

I don’t believe OMC ever published that information (warranty reasons). Between 110 and 120 would be considered very good for that motor.

[putback]

Thanks. It was stored for years. Sprayed it with WD, rolled it over a bit and checked it. I’ll try running it and recheck. See if it’ll give me a 100.

[hugh]

Just don’t discount it on the numbers though. I had a 56 Lark 30 hp with 85# comp. I left it lay for a year as I didn’t think it would run good. Tuned it up one day, put it on my StarCraft and it turned out to be a great runner. Idled good, started good, never died when trolling for hrs. I have another one on the rack now. It is up around 120 and nice looking too. I just don’t have a use for it. Good luck with it.

[beerman57]

I had a QD11 (1950 – 10hp) with only 70psi, but it started right up, idled great AND ran 22-23mph on my 13′ Feathercraft, using the 2 blade brass prop.

[jeff-register]

PutBack,
Spraying the cylinders will give false readings, but run it on the boat for a few hours then check it. If it idles well that is a good sign it is OK,
The two cylinders must be with in 10% of each other to have even wear. The OMC motors don’t run as high compression compaired to Mercs but they last forever using the correct oil mix & a clean fuel system. Two strokes hate dirt as the oil/gas mix carries the dirt straight into the engine.
Take your time & be careful. You will be rewarded!

[putback]

I’m certain it sat for 5 yrs, I suppose it could have been 10+. Turned free by hand, just wanted a bit of lube on the rings / walls before I cranked it on the elec start. Both were dead on 90 psi, three days later both are now dead on 95, encouraging. Looking through the plug holes there is some serious carbon build up on the piston tops. No spark, its full elec including shift. I’m tracing out the wires looking to see if I need to jump a couple at the connector before I pull the flywheel. Time and careful, you bet. I need one of these for my resto hull, in my area took me a year to find one.

[wedgie]

Compression may come up if you run some OMC engine tuner or Seafoam fuel additive through it to clean out the carbon, and possibly unstick any stuck rings.

[crosbyman]

pamper it with a few tanks of xd-100 oil at 50/1

Joining AOMCI has priviledges 🙂

[Author]

Posts