A few questions on inspecting older motors...

When testing compression on used boat motors what is an acceptable reading per cylinder?

What is the reading when new?

And I assume the older the motor (say 20 years and older) will have substantially less psi than say a used 5 year old motor?

Thanks!

A boat mechanic of 15 years told me that on my 1989 Johnson 110hp, the compression should range between low of 125psi and high of 150psi. Also, they should be pretty much even across each cylinder, with no more than 15psi variation.

I don't know if this is the same for all outboards, that was just the info given to me for mine. I know one of our fine mechanics can give more insight.

Wow that seems higher than I thought.


I've been looking at Bass Trackers with mostly 2 strokes and some 4 strokes in the 40 to 90 hp range. That probably makes some difference. I figured around 120 psi new but I really don't know.

I actually went to a local shop today an he showed me what i need to inspect before buying. He said on eveinrude/ johnson i belive ran about 90 and merc's were 120 an 130 in some cases. He said basic rule test and the range shouldnt be more or less 10 so if 3 show 90 or so an one shows 50 run away.

Oh and thanks neal sunrise marine in round rock Great guy,

WOW, I'm impressed that a shop would give you tips like that, great place to shop!

So different motors=different specs. Makes since but that's a huge difference between manufacturers. Hopefully a marine tech will chime in here.

I guess I've been mostly seeing 50 hp Merc's on the Bass Trackers if that makes a difference.

Yes he is talking about compression within 10% of each other because their is so much variance in what normal compression is on different models and brands of outboard motors. Late model 50hp Mercs should be close to 120psi.