Its hard to find good data supporting what an appropriate measurement of cylinder compression should be in older outboard boat motors. I have owned several Johnson / Evinrude outboard motors, a couple of Mercury’s, and at the time of this post one Yamaha four stroke. I have started this post as a repository of my findings regarding cylinder compression and hope to expand it over the years as my habit for tinkering with outboards continues.
Feb 4, 2015 — A number of Yamaha outboard models have been built with different compression in each cylinder for the best. Compression 1998 johnson outboard repair manual for. Oct 7, 2016 — Hello BD members, I have a question about a 2006.
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From my research, cylinder compression values that people say are “acceptable” for low horsepower outboards seem to vary. They not only vary from the reporting individual but from manufacturer to manufacturer. For instance, a fully functional Mercury 25 Hp cylinder compression value may be significantly different than that of a Johnson 25 Hp. Its also rather hard to find where a manufacturer will say the compression range should be from this psi to that psi. What I have found unanimously is that the relative compression values between the cylinders should be within 10%. This source says modern two-stroke outboards should be in the 90 to 105 psi range. However if you watch the end my video below you will see that my 1979 Evinrude 6 Hp cranks easily cold ( 2 pulls) with 60 psi compression This source says the older two stroke outboards below 25 Hp should be between 75 and 85 psi. This sourceseems to confirm with my understanding that 60 psi in older outboards can sometimes be OK. Bottom line, if I was buying an older two-cycle outboard I would probably want to see compression values above 75 psi ESPECIALLY if I could not see it run.
Four cycle outboard motors produce much higher compression than two cycles. However, four cycle engine cylinder compression should still be within 10% cylinder-to-cylinder.
Older two cycle outboards, especially low horsepower, can run fine around 60 psi however above 90 psi is optimal. Four stroke/cycle engines produce much higher compression. This site cannot be reached chrome. The main thing to check is the difference between cylinder compression. Cylinder-to-cylinder compression readings that differ more than 10% reveal trouble.
You hear people talk about compression numbers.Do any of you people know what a newer motor compression numbers are
Johnson 150 Outboard Compression
A newer 25hp Merc will have around 145 to 150 #'s.I have purchased older Mercs & have had compression readings around those numbers also.Some times the rings may be stuck, but a little Marvel Mystery oil will take care of that if not too bad.I have a 1974 85hp Merc that the numbers are around 160# with no motor work done.