Compression for older OMCs

[arcadiainc]

Presently my collection has a 1958 18 hp Johnson, a 1955 5.5 Johnson and a 1970 9.5 Johnson. I bought a decent screw in compression gauge , however I can not find compression figures for these engines. The 9.5 hp has 70 and 72 psi. Think it is low but not sure. The engine runs well enough. Does any one know what the compression on these engines should be to be considered in good/excellent condition?
Thank you in advance.

 

[lindy46]

Re: Compression for older OMCs

Presently my collection has a 1958 18 hp Johnson, a 1955 5.5 Johnson and a 1970 9.5 Johnson. I bought a decent screw in compression gauge , however I can not find compression figures for these engines. The 9.5 hp has 70 and 72 psi. Think it is low but not sure. The engine runs well enough. Does any one know what the compression on these engines should be to be considered in good/excellent condition?
Thank you in advance.

70psi is a little low for a 9.5 but probably acceptable. As long as readings are within 10 percent of each other. My 1955 Johnson 5.5hp runs about 75psi and runs good. For the 18hp if it's running 95psi+ should be good. Might want to soak the pistons in Seafoam overnight to free up the rings. I picked up about 10psi on my 5.5hp by doing that. Then run some Seafoam in the gas to clean up the cylinders.

 

[kenmyfam]

Re: Compression for older OMCs

Compression testers vary somewhat in their readings and accuracy. Can you compare results with another tester ??
Generally above 100 psi and all cylinders within 10% of each other is a rule of thumb.

 

[steelespike]

Re: Compression for older OMCs

70psi is a little low for a 9.5 but probably acceptable. As long as readings are within 10 percent of each other. My 1955 Johnson 5.5hp runs about 75psi and runs good. For the 18hp if it's running 95psi+ should be good. Might want to soak the pistons in Seafoam overnight to free up the rings. I picked up about 10psi on my 5.5hp by doing that. Then run some Seafoam in the gas to clean up the cylinders.

Don't be confused by "soak the pistons".Just do a good decarb as described on the Sea Foam container.Sea foam and similar products seem to be one type of the few "additives" that really work.

 

[arcadiainc]

Re: Compression for older OMCs

Thank you for the replies. I have Seafoamed the 9.5. Started with about 60 psi and now a little above 70. My first time with the product and did not know what I was doing. Think I got a little carried away. I have yet to check the other two motors. Miserable weather.
Thanks again

 

[Doncc]

Re: Compression for older OMCs

Thank you for the replies. I have Seafoamed the 9.5. Started with about 60 psi and now a little above 70. My first time with the product and did not know what I was doing. Think I got a little carried away. I have yet to check the other two motors. Miserable weather.
Thanks again

Terrific suggestion and tips as a first start to improve compression BEFORE taking the engine apart! Seafoam sounds like a terrific product.

 

[JB]

Re: Compression for older OMCs

I am one who doesn't buy 100psi as a "rule of thumb". Guages, technique and conditions vary too much to put any arbitrary number on psi as minimum acceptable. Makers won't do it, though OMC usually asked for 100psi or better they also asked for a warmed up engine, which most seem to have forgotten.

The probability that two, three, four or even six cylinders all fail by the same amount (within 10%) at the same time seems so remote to me that I ignore the possibility. I consider the purpose of a compression test to find out if any one cylinder is failing. The only exception to this is a twin with a blown gasket between the jugs.

A good decarb will usually raise compression numbers on almost any engine; if for no other reason because the engine is fully warmed up.

I don't worry about any rope start outboard that compresses over 50psi, an OMC V6 the makes over 60psi. . . actually, any outboard that puts all cylinders in a 10% range.

 

[mchin]

Re: Compression for older OMCs

I have several OMC motors, and 3 of them are 9.5 evinrudes/johnsons and 2 of them have only read between 65 and 70 psi of compression. Both motors run great and never miss a beat. Now the 9.5 hp motors in my opinion are no power houses, mine get me up on plane, but are not as powerful as a modern motor. The 5.5 are also not the fastest motor, but man are they reliable.

My favorite is the 18 hp, they are so easy to work on, very simple to maintain and for their weight, they haul butt. On a light boat, you'll fly, on a heavier boat, I think you'll find the 18 to be very capable. On a 12 foot overweight fiberglass sears gamefisher boat, I was able to hit 26 mph on a gps, with a dolefin, without the fin I could go about 2-3 mph faster, but the boat would bounce like crazy.

You've got some great motors there and with care, should run fine. Watch your mix ratio, I think the older motors need 16:1 ratio.

 

[jbjennings]

Re: Compression for older OMCs

I don't have the years of experience that some of these guys have, but I personally have a little different viewpoint on compression which is based not only on my findings but also on information I've obtained from trying to pick the brains of other guys who do have a lot more experience than I have. I'm specifically referring to '50s OMC outboards which is what I like the most.

Here's some compression readings that I've found to be pretty good indications of a motor's internal condition:

'53-'54 johnson/evinrude 25hp 115-130psi Excellent
110-115psi good
<100 not good, wouldn't want it

'57 and '58 18hp johnson/evinrude: 120-135psi EXCELLENT!
110-120 Good
95-110 Borderline, but runs well
<95 wouldn't want it, but may run

'56 15hp above 110psi excellent
95-110psi good
<95 ?????

'53,'54 15hp evinrudes Some folks report a little higher compression on these for some reason than the '56 models, my '53 has 110lbs. and runs good, but I suspect that 110lbs. may be on the low side of the good or even borderline.

'53-'59 10hp all of my decent ones run about 85psi
90-110psi---excellent
80-90psi--- good
75-80 ---borderline
<75psi--wouldn't want that motor, although down to 65
or 70psi it might run o.k.

'50's 7.5hp similar to the 10hp, maybe 5lbs. less on average

Whether or not your compression guage is accurate or if you can perform the test properly is up to you, I'm just giving what I've found. I think that compression figures can be really useful in giving you an idea of how the motor is going to run for you when it's properly tuned. A motor that hasn't been overheated or run low on oil, but just run the heck out of for years and years can, from what I've seen, have very close to the same compression in both cylinders. But it will be lower compression than a lower hours motor and be harder to start and have less power and not idle quite as smoothly. You probably wouldn't even notice the less power on top end, but would notice the starting and idle characteristics. When I get a "new" motor, I do the comp. test on the cold motor with no addition of oil. I like to turn the flywheel over a few times with the plugs out to make sure everything is turning nice and smooth before I test it and if not, use some light oil until it is. BUt if I have to do that, I don't rely on my compression results until I've ran the motor a while and let it cool down overnight.

Just some info. and ideas,
JBJ

 

[coolguy147]

Re: Compression for older OMCs

well my 1959 evinrude 10hp runs like a dream. i got 95 psi in both cylinders dead even. again its a pull start. im sure if i put like a drill on the top it would go over 100psi. but im again also sure that while running its running above 100psi


anyways im sure i got a good engine