Compression question

[Jackson65]

Ok, guys. It’s time for everyone’s favorite topic: compression. I recently acquired a 58 Evinrude 18 hp with no magneto, flywheel or rewind. I located and purchased all the missing parts (I have some coils, plug wires, points and condensers). Anyways, I tested the compression and the top cylinder hit 130 psi and the bottom cylinder hit 110 psi. So of course, my question is, should the fact that those readings differ by more than 10% concern me? I was very pleased to see readings above 100 psi for both cylinders, but I would have been happier if the readings were closer.

Thanks,
Jack

[billw]

The bottom cylinders on those engines tend to ingest water. You are lucky that thing isn’t seized up, like three quarters of the ones I have had. I would bet that the rings on the lower piston are at least partly stuck. Pretty much every 1950s 15 or any year 18 that I have ever taken apart has had rings stuck, to some extent, even if the engine seemed to turn over well before I took it apart. (The same is true for Mercury Mark 20s and 25s, by the way.) That being said, 130 is excellent and 110 is pretty darn good. Some people are fans of engine tuner, sea foam, etc. Maybe you should do the mag, carb and give it a good run with one of those mechanic-in-a-can things, and then retest. I doubt any harm would be done in trying.

Long live American manufacturing!

[jimk]

I have a 1957 that sat for a few years that had about 10 psi difference but after a few trips out is was within 2#. One of my best running motors!

[Jackson65]

That was what I was leaning towards. Get her together and see how she runs. Run some Seafoam through her and see how things go. I know compression readings over 100 are definitely desirable. I am very hopeful for this ol gal, but then, I am always hopeful I can bring them back to life. Thanks for your thoughts on this. I was not aware that that lower cylinder was so likely to have ingested water, but then I am a newbie. Some knowledge only comes through experience.
Thanks!

[crosbyman]

of all the oldies (15 or so …. JWs up to FD’S) I fixed up none ever had over 90 psi and…. they all work fine . Could they have run better… maybe maybe not.

Joining AOMCI has priviledges 🙂

[billw]

I have had a few Fastwins that had about 90 PSI. Not much I could do about it, because the cylinders were worn. They started easy, sounded great and idled down to nothing. Where the low compression showed up was at higher speeds: They just were not as FAST as I knew they were capable of.

Long live American manufacturing!

[Rusted Junk]

You can put a shot of Miracle Oil in the cylinders, let it sit for a couple days. Then pull the plugs and clean and gap them and turn the motor over a few times.Mix about an ounce of Miracle Oil to one gallon of your two stroke mix and take it out and test on it, those rings should come right back around and be fine.

[Jackson65]

Sorry Bill, I have been so busy I hadn’t taken the time to reply. I had a ’55 Evinrude 15 horse restored a little over a year ago (that is when my OMC addiction kicked in) and it was sitting at 90 lbs. per cylinder before repairs. Runs great and pushes a jon boat about 19-20 mph, and as you said idles so low I have to pull the choke or disconnect the gas line to kill her. I love these old motors.

[Jackson65]

Rusted Junk,
I use Miracle Mystery Oil in my car, lawnmowers and such, so I know it is good. Do you prefer it to Seafoam?

Jack

[fleetwin]

You have already purchased the needed parts, so have nothing to lose from putting it together, running it and retesting….

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