Compression Test

[lsufansr08]

Ive got a 40 Hp merc 3 cylider. Been haveing trouble starting, and with it missing. it also overheats. So i did a compression test. It shows 72-75 lbs on all three cylinders. Seems to me that it should be higher, but is it strange for all three to have the same amont of low compression. Just brainstorming, hopefully some one can help me with this problem.

[Ray]

It does sound low. May have ran lean on 2 cycle oil if they are all the same.
It should start and run, but won't have much power.
You could also have bad reed valves if it is low.

[gerald]

It could be that the gauge is reading wrong.

If you know someone with an outboard......try checking there compression with your gauge.

Go to the Merc web site and see what the compression should be on a new engine. They only show information on 2012 or 2013 engines and your's is probably a few years old......but should be about the same.

[lsufansr08]

Thanks ray. That's way I was afraid of. I'm sure that's a pretty costly fix. Appreciate you always keeping up with the post

[JAdams]

It's prolly gauge is wrong I find it odd u would blow all 3 cylinders and have even compression on all 3 40 merc between 110 and 125 as long as there all even ur good

[JAdams]

Thermostat sticks in those a lot