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Tohatsu decompresser

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Pritchy

New member
Hi
I just did a compression test on a Tohatsu 20hp 2010 model I just purchased.
I nearly had a heart attack when I couldn't get anymore than 50 psi out of each cylinder.
I started reading the service manual on how to change a head gasket when I came across this about decompression for easy start up.
Could this be the reason for such bad psi and if so can it be removed so I could check the compression without it in place ?
Thanks
 
Last edited:
Year is irrelevant; You probably have an MFS20C.

The decompressor is a weight on the camshaft, which flies out of the way at running speeds. It engages below say 500 RPM, and disengages up around 800 RPM. So, if electric start, you can pull plugs and crank, and get a pretty good value for compression. If rope starting, use the compression values from the Factory service manual: about 60 with decompressor, up to about 120 or 130 with full compression. The test should be made on a warm motor, with throttle blocked open, and the pressure will climb each compression until reaching the max. Therefore, 50, while not stellar, is not bad.

The big thing: It should be even between cylinders... not vary more than 10% (ideally not more than 5%). If it seems low, add some oil to the cylinder and re-test. If it climbs, the rings are leaky. If not, the valves are leaky.

What symptoms are causing you to check compression?
 
pvanv said:
Year is irrelevant; You probably have an MFS20C.

The decompressor is a weight on the camshaft, which flies out of the way at running speeds. It engages below say 500 RPM, and disengages up around 800 RPM. So, if electric start, you can pull plugs and crank, and get a pretty good value for compression. If rope starting, use the compression values from the Factory service manual: about 60 with decompressor, up to about 120 or 130 with full compression. The test should be made on a warm motor, with throttle blocked open, and the pressure will climb each compression until reaching the max. Therefore, 50, while not stellar, is not bad.

The big thing: It should be even between cylinders... not vary more than 10% (ideally not more than 5%). If it seems low, add some oil to the cylinder and re-test. If it climbs, the rings are leaky. If not, the valves are leaky.

What symptoms are causing you to check compression?
Click to expand...
HI,
Thanks for getting back and explaining ....much appreciated.

I traveled 7 hours to pick up the outboard and after already buying two duff used outboards i was a little apprehensive. It looked great in the tank and i left very happy. When i arrived home and started playing with it i removed and checked the plugs . The compression tester was next to me so i gave it a try and that's when the panic set in.
After running it for a while i noticed white smoke and thought low compression ,white smoke -head gasket.
After worrying most the night i went back out and did another compression test with throttle open,long pulls etc and now i'm getting 90psi.
I have checked the thermostat and the water pump and they are like new . I have never seen an 8 year old outboard in such good condition.
It starts first pull really easy and runs sweet .
So according to what you say, my 90psi is pretty good ?
Just the white smoke to check on again then i guess.
 
The white smoke is likely just steam. That motor has a metal head gasket, so unless it went through a severe overheat, you are probably just fine. Compression sounds fine.
 
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