1951 Johnson 5 H.P. has Low Compression on both Cylinders

[hugr20]

I have a 1951 5 H.P Johnson model # TN 27, that I just bought and I checked the compression on both cylinders and it is only about 20-25 on each. I pulled off the head and the cylinder walls look good. The motor shows no signs of overheating. Why is there no compession? Could it be reed valves or new rings?

 

[KillerJello]

Re: 1951 Johnson 5 H.P. has Low Compression on both Cylinders

could it possibly be a bad head gasket between the two cylinders? Im not an expert on boats, but with a car, if two side by side cylinders have low compression, its usually a bad head gasket...

 

[K Hultgre]

Re: 1951 Johnson 5 H.P. has Low Compression on both Cylinders

I'm not an expert but I do know the older motors did not have very high compression even though this sounds quite low. I'll keep watching to see if anyone posts the compression range you should have.<br /><br />Have you tried to squirt oil into the cylinders and perform the compression test again. It is possible the rings are stuck.

 

[alcan]

Re: 1951 Johnson 5 H.P. has Low Compression on both Cylinders

Hi hug<br />Odds are that this motor has been sitting up for most of it's life (not being used). These motors use a 24:1 fuel/oil ratio useing mobiloil outboard oil. They also give instructions for a 16:1 ratio useing reg nondetergent 30wt motor oil, IF OUTBOARD OIL IS NOT AVAILABLE. Anyway do to the fairly high amount of oil being used in these engines the rings tend to get stuck ( compressed to the piston) during long periods of rest. Most of the time all that is needed to wake them up (loosen the rings) is a fresh dose of mixed fuel in the cylinders. If this does not work soak the cylinders with a little Seafoam. I find this very common with these models. Also they don't have real hi compression in the first place. Let's say anywhere from 75-100PSI.

 

[hugr20]

Re: 1951 Johnson 5 H.P. has Low Compression on both Cylinders

Thanks for the info guys, <br />but the motor still has very low compression although I can get it started. Also is not pumping water, the pump housing and impellor have seen better days. Looks like I should throw this one overboard!

 

[G DANE]

Re: 1951 Johnson 5 H.P. has Low Compression on both Cylinders

Hi hug 20<br /><br />Are u sure your gauge is right, i dont think it will start with only 20. Try another gauge.

 

[hugr20]

Re: 1951 Johnson 5 H.P. has Low Compression on both Cylinders

I tried a different guage after I ran the motor and sure enough the compression is higher. About 60 lbs on each hole.