Poor engine compression, '55 Johnson 25HP

[joaklay]

I have a '55 Johnson 25HP that I bought as a "parts motor" as the seller knew it had problems.<br /> He said it ran "about 2 years ago" (who knows?) and that it lacked high speed power. He had the carb apart too and it needs new seals, gaskets.<br /><br /> So....I'm an auto tech but I'm generally inexperienced so far with 2 cycle engines. I checked this engines compression and it's not good.<br /> What are the first things to initially check? Is it best to first run the engine before checking compression assuming that maybe it has gunked rings? (Guessing)

 

[Paul Moir]

Re: Poor engine compression, '55 Johnson 25HP

What were your readings? You may not be aware that compression reading on these engines are normally very low. If your readings are within 10% of one another, you're probably good. A reading of 90psi or more would be considered excellent. Carboned rings can cause low compression, and cleaning it out with a decarb fluild is always a good idea. Pour some directly into each cylinder at mid point (if one's all the way down it will just drain out the exaust and intake ports).<br />If in doubt, pull the head and make a visual inspection of the cylinder walls. Head gaskets are pretty cheap, and it's easy to pull the head off of a 2-stroke. <br /><br />The most common problem with those outboards are cracked ignition coils, which arc over at (somtimes) higher rpms. If they're original, I guarentee they're shot. New ones are cheap and availiable. NAPA autoparts even has them. Hint: a beefy harmonic balancer puller works as a flywheel pullter. Anything that pulls from the outer edge of the flywheel will break it. 'Tapping' will not get that one off; the nut is tightened to 105ft/lbs.<br /><br />A good carb soak, blow out and reassembly with new needles, float and seat is a good idea. The old cork floats have a tendency to sink.<br /><br />Hope some of this helps!

 

[R.Johnson]

Re: Poor engine compression, '55 Johnson 25HP

Your dealing with a 49 year old engine here, so you will need to go through the whole system, plus deal with that old pressureized fuel tank. You said the compression is low, how low? I would pull the head, and check cylinder walls first. If they are OK, here's what your dealing with. New points, condensers, coils, plugs, water pump, gearcase seals, plus that fuel tank. That 25 was a pretty rugged old motor. I bought one of those new, my brother, and I put a lot of hours on that thing.

 

[joaklay]

Re: Poor engine compression, '55 Johnson 25HP

OK, thanks for all the replies so far.<br /><br /> Maybe it's really not so bad afterall or perhaps my 25 yr old compression tester isn't accurate anymore either?<br /> The '55 25 HP has 45 lbs per cylinder (best I can get). I also have a '54 25 HP and it has better compression at 65 lbs per cylinder. I've looked into the '55s cylinders as best possible with a mini light and don't see damage.<br /> I then checked another old outboard (my '54 5.5 Johnson) that's been in the family since new, always starts right up and runs perfect, it has 50 lbs per cylinder.<br /> Are my readings then normal for these old motors or is it most likely that my guage needs replacing? Obviously the '54 25 HP has the best compression of these three. <br /> Even with auto engines when used, my old compression tester reads much higher and not seemingly abnormal to me either but they're also 4 strokes.<br /><br /> As for the other ideas mentioned here, I'll definately check it all out when weather warms up enough for me to do that (spring).<br /> I have a new carb kit with a new float and the original dual line '54 gas tank for the 5.5 still works so fuel delivery won't be an issue here.

 

[G DANE]

Re: Poor engine compression, '55 Johnson 25HP

Hi joakley<br /><br />I think your gauge reads low. Try it on your car, lawnmover or others. If everything has low numbers guage is wrong. Compression below 60 ies normally makes it poor running.

 

[Paul Moir]

Re: Poor engine compression, '55 Johnson 25HP

Since most of us are using compression testers in the far lower end of their range; they tend to be pretty inaccurate there. Try to borrow one from someone else for comparison. Sounds to me like your reading low.<br />If you want to be sure, pull the head. It is a simple task on a 2-stroke, and a new head gasket is pretty cheap, and it gives you a chance to clean out the water jackets. If the bore is scratched and gouged, or rusted and pitted, it's not going to perform.<br />Stuck rings on an old 25hp is not unheard of either. I've got an almost identical mechanically later 28hp, and I never did extract the rings from the pistons on my parts motor. Years of bad/wrong oil, no thermostats (like your engine) and 'nursing' did that engine in. The top rings were so badly coked, blowby gasses were igniting the charge in the crankcase until the side of the crankcase finally blew out. They patched the blowout with JB Weld and continued to run it until the top bearings failed from lack of lubrication. The very blue rollers from the top crank bearing fell out of their cage when I removed it.

 

[joaklay]

Re: Poor engine compression, '55 Johnson 25HP

OK, removed the head and the cyl walls are scored, can feel grooves (probably poor lubrication) so that'd explain the 20 lb difference between my 2 old 25 HPs.<br /> I have a line on another powerhead so I'll look into that one as it's suppose to have good compression.