compression 40 psi after changing head gasket tohatsu

Michawilden

Petty Officer 2nd Class
Joined
Apr 4, 2015
Messages
153
I'm repairing a 2019 6 hp 2 cyl 4 stroke tohatsu outboard. Same powerhead as the 8 and 9.8 hp tohatsu and mercury I believe. The motor was running and starting really good. Started in 1 pull. Problem is that the motor got water in the oil. Quite a lot. I figured that it might leak at the head gasket so I pulled the head. Did not see any damage but still replaced the gasket as well as the lower powerhead gasket. Checked for warping on the powerhead and cylinderhead. Could not see any warping so I cleaned the head and installed it with a new original gasket.
Set the timing again with the timing marks and figured It would start right up but first pull did not create as much compression as before. Compression test gave only 40 psi on both cylinders with the carb at WOT.
I will check and set the valve clearance. I did not change it so I figured that would just remain the same. Next step is that I pull the head again and check but what did I miss?
This motor does not have an automatic decompression I believe.
 

Michawilden

Petty Officer 2nd Class
Joined
Apr 4, 2015
Messages
153
MFS6B. You can not find it on boats.net. Thats only the single cylinder. Im from europe.
 

km1125

Chief Petty Officer
Joined
Aug 10, 2016
Messages
515
I'm confused on a bunch of levels. The MFS6 is a single cylinder engine (at least I thought they all were). And if it is a four-stroke engine there is a compression-reduction mechanism that will reduce the compression pressure at low RPMs. The spec (IIRC) is around 50 or 55 PSI, so you're not far off. You'd have to disable this mechanism or spin the engine over 200 RPM to get a more accurate real compression test.
 

Michawilden

Petty Officer 2nd Class
Joined
Apr 4, 2015
Messages
153
Yeah, I can understand that you're off. However, It is a twin cylinder engine. Same as the 8 and 9.8 hp.
 

pvanv

Admiral
Joined
Apr 20, 2008
Messages
6,555
Yes. Euro version 2 cylinder. No decompressor. Should have higher compression.
 

cyclops222

Lieutenant Junior Grade
Joined
Mar 21, 2024
Messages
1,232
Supplied you with the wrong gasket ? If you have the old gasket ? Lay it over the new one to see if they have identical holes I always compare a new part very carefully to the old part.
 

Michawilden

Petty Officer 2nd Class
Joined
Apr 4, 2015
Messages
153
I will check. I bought the 8 hp gasket since I could not lookup the 6hp parts. Maybe the 6 hp has a different bore. I will update soon.
 
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