Tohatsu four stroke 5hp upgrades

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Ozzy RZ

Cadet
Joined
Jan 12, 2011
Messages
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Firstly, Hi, I have just joined this forum and I have just got my first 4 stroke Tohatsu motor.
I am a bike mechanic and tuner by trade and love to tinker with motors.

My new Tohatsu is a 2010 Model - MFS5B
Serial Number - 040541AX
So, from what i have seen stated in the forums, if i buy the MFS6B carburetor, this will make my 5hp turn into a 6HP?

In regards to the New Tohatsu 5hp or 6hp four stroke engines,

Has anyone checked the head squish measurement and flowed the head on these motors ? I know that on 4 stroke motor bikes engines we run about 0.9-1mm squish clearence to improve compression. And in most cases, most 4 stroke heads can be made to flow better than stock.

What your thoughts?
Kindest Regards
Matt
Ps, We have 95-Ron fuel at every gas station here in OZ! .
 
pvanv

pvanv

Admiral
Joined
Apr 20, 2008
Messages
6,574
Re: Tohatsu four stroke 5hp upgrades

Yes, the carb will up your output to 6 hp.

As a dealer, I haven't milled these heads for more compression, nor ported/polished them, but it probably would help some. How much are you expecting from this little 1-lung motor anyway? When tinkering with the combustion chamber, keep in mind that the pin and rod ends run directly on plain journals, and are not needle bearings. BTW, 95-RON (research octane) might well be similar to our 89 R+M/2 product, since Motor octane is always a lot lower than Research octane.
 
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Ozzy RZ

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Jan 12, 2011
Messages
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Re: Tohatsu four stroke 5hp upgrades

Hey thanks Paul.
I might check the compression and squish clearence today.

So your a dealer?
Would you be able to supply me with the 6hp carb/base gasket/head gasket and a few aother little bits if i strip it down?

Regards
Matt
 
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Ozzy RZ

Cadet
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Jan 12, 2011
Messages
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Re: Tohatsu four stroke 5hp upgrades

Lads,
I measured the squish band this morning and the measurement is 1.20mm
So, that means you can machine 0.30mm from the head surface to increase compression. This basicaly gives you more torque.
That will have the squish set at 0.90 which is good for all fuels (except those containig ethanol).
If you run a good premium fuel or high octain, you can set your squish at 0.80mm.
I set all of my race motors that we build at that measurement. But you do need to run good fuel all of the time.
My advice (for those interested) is to machine the head to 0.90mm
I have not inspected the head porting and manifolds yet, but i will let you know what i find soon. (I have never seen a 4 stroke head that didnt benifit from some form of porting or flowing work)

Regards Matt
 
O

Ozzy RZ

Cadet
Joined
Jan 12, 2011
Messages
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Re: Tohatsu four stroke 5hp upgrades

Ok boys, Paul has sent me a 6hp carb for my 5hp 4 stroke. It has not arrived yet, but i was wondering if anything else needs to be changed before it arrives. Eg, propeller, ignition timing?

Regards
Matt
 
Sea Rider

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Re: Tohatsu four stroke 5hp upgrades

When final adjustements/modifications are done, make a wot test with a inductive tach to check max rpm obtained and play with prop pitch if needed to match max 6000 rpm for best top speed.

Happy Boating
 
pvanv

pvanv

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6,574
Re: Tohatsu four stroke 5hp upgrades

Ozzy RZ said:
Ok boys, Paul has sent me a 6hp carb for my 5hp 4 stroke. It has not arrived yet, but i was wondering if anything else needs to be changed before it arrives. Eg, propeller, ignition timing?

Regards
Matt
Click to expand...
Timing is not adjustable, so proper propping is the only tuning you would normally do on these motors when changing carbs.
 
O

Ozzy RZ

Cadet
Joined
Jan 12, 2011
Messages
7
Re: Tohatsu four stroke 5hp upgrades

So is there a 6hp prop that you guys use and recommend?
I looked at the brochure and nothing was mentioned about props.

Regards
Matt
 
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