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Tohatsu M8B upgrade

F

fvee91

Member
Hi All
I have a 8hp Tohatsu 2 stroke, early 90's model. It is a bit underpowered for the boat we are using and I would like to get a bit more out of it. I notice that the borexstrok is the same on the 9.8, so what is the difference between the two? is it only in the carb or perhaps higher compression pistons as well?

Thanks
 
The blocks are different, and vary by serial number.

No matter what, if you have a compatible block, you would need 3K9032000M CARBURETOR, 9.8 MF/EF MODEL $144.80
 
Hi Paul
Serial number is 52235 M8B.
If the carbie is compatible, do you have a price on freight to Sydney Australia. Also while we are in there, is there any benifit to changing the reed valves or adjusting the ignition timing? Or any other modification to improve top end power.

Thanks
Daniel
 
You have a different block, so no upgrade. The reeds are best left stock for all-around performance. Same for timing.
 
Hi Paul
I have done some more research and came to the same conclusion, it seems that the 9.8's had a better exhaust port which allowed more air through a bigger carbie. Also I have found reference that the 9.8 had a different head which gave her more compression. Can you confirm? Also do you know how much more.

Thanks
Daniel
 
Yes, different block, and different head. Unless you are considering doing a wholesale upgrade of everything, from an old donor motor, I wouldn't get into it. Just would not be cost effective.
 
Hi Paul
All im really considering doing at this stage is pulling the head, shaving it down a bit and putting it back. I understand that doing a complete swap to a true 9.8 would not be feasable. I am hoping that you can give me any difference in compression between the 8 and 9.8, and your general thought on going higher

Thanks
Daniel
 
For overall reliability and runnability, you will be fine at stock. Increasing compression will require premium fuel. HP increase will be marginal, pull starting will be harder, and idle will suffer. I suppose you could mill 0.010" off, and maybe, maybe, end up at 8.5 hp on the dyno, if everything else is like new.
 
Michael, You have a different block from the 9.8, so no economical hp upgrade. You would need a carb, head, block, etc. And while at it, do the bearings and rings. Add in labor... Way more cost than a new 9.8.
 
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