2013 Merc 20hp 4 stroke - Low Compression??

[Elliomcmh]

Hi,
i have a 2013 Merc 20hp 4 stroke. I decided to check the compression after some vibration at trolling speed. I got 55-60psi on both cylinders, is this normal? I remember my old evinrude compression being significantly higher.

i checked the compression on a cold engine, throttle in the idle position, and had the spark plug in the cylinder I wasn’t checking, not sure if this is the correct procedure.

Thanks for the help!!

 

[racerone]

Throttle must be at least half open so test it again.

 

[Elliomcmh]

I’ll check it at lunch, and post the results.

 

[Elliomcmh]

Ok I checked both cylinders with the throttle at 50% and 100%, I’m still getting 55-60psi on both cylinders. I also removed both plugs and tested each cylinder that way at 50% and 100% throttle, and it was still 55-60 psi. Engine was cold, 55 degrees outside.

 

[racerone]

Something is wrong with your gauge or you have serious engine issues !

 

[Elliomcmh]

it’s a new gauge I got off of amazon. I could try buying another gauge tomorrow. Would it idle fine with a little vibration at 1/4 throttle and run fine at top speed if the compression is this bad? It’s on a Tracker Grizzly 1448 and runs 22 mph with a 10p prop.

 

[Elliomcmh]

Update: I just tested the compression on my lawn mower, the spec is 160psi and the gauge was right at 160psi, so I guess the gauge is good???

 

[racerone]

Some trouble shooting needs to be done on that motor.

 

[Elliomcmh]

I just did some research and it looks like some of the Tohatsu made mercury small outboards have an automatic compression release, I wonder if that is why it runs ok but has low compression.

 

[Elliomcmh]

Does anyone know if this is true?

 

[Alumarine]

If it is a Tohatsu you might want to ask about the compression release in that area.
It would not run well at high speed if the compression was really as low as your test shows.

https://forums.iboats.com/forum/engine-repair-and-maintenance/honda-tohatsu-nissan-outboards

 

[raczekp1]

try compresion test with only 1 sparl plug out at the compresion test