Interesting compression results, question.

jahughes · Jul 30, 2005

1973 Johnson 85HP. Compression on 2 and 4 is 140. Compression on 1 and 3 is 150. Motor is new-to-me, I'm guessing that one side (1,3) may have been rebuilt/honed/ringed, or is there another explanation that one of the resident masters might be able to offer? Can anyone tell me if this uneven compression pattern might cause an operational problem? For the few seconds I've been able to get it to run (also working an intermittent spark/no-spark problem) it sounds fine, better than fine actually, pretty dang nice really. I'm probably going to need to buy a stator for my ignition problem so want to make sure compression is not going to make that a waste of money, thanks! Jason

llfish · Jul 30, 2005

Re: Interesting compression results, question.

Usually looking for less than a 10% difference in compression. Someone will come a long and give you a better explanation. Good luck

jahughes · Jul 30, 2005

Re: Interesting compression results, question.

Well, I'm in range then, other comments/info/opinions still welcomed, thanks Larry! J

Goodoleboy · Jul 30, 2005

Re: Interesting compression results, question.

well...Iam hardly a master of anything. Unless we are talking about my beatle hound. My opinon about your compression is that you are in fine shape, everbodys motor should have such high numbers. Your intermient spark issues can best be solved by following the trouble shoot section in your manual. Luck to ya!

jahughes · Jul 30, 2005

Re: Interesting compression results, question.

Thanks Goodoleboy, I think I'm on a good track with troubleshooting the ignition, have another post I just put up on that if you have any ideas, and thanks! J

Solittle · Jul 30, 2005

Re: Interesting compression results, question.

When you get it running add a decarb to your to do list.

jahughes · Jul 30, 2005

Re: Interesting compression results, question.

Thanks SoLittle, I was thinking the same, maybe one side is low, not the other high, eh?

G DANE · Jul 30, 2005

Re: Interesting compression results, question.

Both are very high, but that depends of gauges. Manual advises to compare banks on V4 when looking for equality, like you did. Of coarse all 4 ahould be within range. Was this warm or cold ? Both banks do not always run at exactly same temperature.

jahughes · Jul 31, 2005

Re: Interesting compression results, question.

Thanks GD, this was cold, haven't got it running yet, working an ignition issue. I may be overthinking/overworking this since they seem to be within reasonable tolerance for a cold engine that might also need a decarb, but for my own education can you clarify on comparing banks? Do you mean compare within the bank (1 vs. 3, and 2 vs. 4) or between banks (1,3 vs. 2,4)? Gracias, J

G DANE · Jul 31, 2005

Re: Interesting compression results, question.

Compare within banks - compare 1 to 3 and 2 to 4, as you did. Difference between banks is often seen on healthy motors too, especially on cold readings.

Dhadley · Aug 1, 2005

Re: Interesting compression results, question.

Look at the heads and make sure the heads are a matched set. The port head will have a casting number thats one numer before the starboard head. If they are not a matching set, find one to match the port. If they are a matching set then someone may have cut the starboard head. Not good.

jahughes · Aug 1, 2005

Re: Interesting compression results, question.

Thanks guys, pardon my ignorance but what does "cutting" the head mean? Not familiar with that term. I'll check the casting numbers as well. Thx! J

Interesting compression results, question.

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