Compression question.

[LIGHTFOOT]

I have a 1973 Johnson 20 HP (20R73A). Does anyone know what the compression on this motor is? Thanks for your help.

 

[AlTn]

Re: Compression question.

even...no more than 10 percent variation...60 lbs. very minimum < in my experience> to run an idle

 

[archcycle]

Re: Compression question.

I think he might be referring to the compression ratio?

 

[LIGHTFOOT]

Re: Compression question.

60 pounds or more with no more than a 10% variation?

 

[AlTn]

Re: Compression question.

yep

 

[LIGHTFOOT]

Re: Compression question.

yep

Thanks for the info!

 

[jbjennings]

Re: Compression question.

Good running 25hp OMC's from that time period will have about 120 psi +/- 15psi of compression on MY gauge. My suspicion is that they had about 125 or 130psi brand new, although no compression figures for any motor were ever published by OMC. Since everyone's gauge seems to read different from mine, I'd go with the less than 10% difference method as mentioned. Personally, I don't think you'd ever get it started with just 60psi of compression, or 70psi. If it were really that low and one was actually successful in getting it started, there's no way it would idle properly.
JUST MY OPINION!!
JBJ

 

[JB]

Re: Compression question.

That is a rope start engine, JBJ. In my experience they compress much lower than electric start engines when in excellent health unless Gov. Schwartzenegger is jerking the rope.

 

[1946Zephyr]

Re: Compression question.

I pulled over my '59 35hp and got 120 psi per hole. As for the old 18's and 20's......I think at least 100 per hole is good. 60 psi is what I generally got on my HD or JW's. 70+ on a 5.5 and so on. But yea, if you're even on both, then that's sign that your pistons are in good shape and the rings are working right. A dramatic difference typically pooints to piston scoring and/or stuck rings.