Carb modification 4hp to 6hp Tohatsu

[Grad]

Hi,

I was trying to reply on another thread at this forum but that one is closed. Still I would like to share some awesome experiences with my modification 4hp to 6hp four stroke.

Old situation: STI280 @4hp -> 10.5 mph
New @6hp -> 15.7 mph

I just bought the main nozzle (3AU032520) Main jet (3AU044680) and Pilot jet (3R1032540). I'm not sure if you need the last one but it's only $7.32. The total of all these nozzles are $ 24.38.

If you put these in the carburetor of the 4hp it won't work because of the small venturi. By removing the fuel valve (the choke valve is not easy removable) you can get to the venturi which I have increased with a table drill to 15mm (14mm will do). Everything works very smooth now.

I also have changed the prop from 7 to 8" pitch.

So you don't need a total new carburetor, by increasing the venturi with a drill you get excellent results.

 

[Sea Rider]

Re: Carb modification 4hp to 6hp Tohatsu

Welcome to Iboats,

You still need an induction tach to check max achieved rpm at wot to see if inside min-max factory stated wot parameters for that engine, why have you changed pitch ? were you over revving ?

Happy Boating

 

[Grad]

Re: Carb modification 4hp to 6hp Tohatsu

Welcome to Iboats,

You still need an induction tach to check max achieved rpm at wot to see if inside min-max factory stated wot parameters for that engine, why have you changed pitch ? were you over revving ?

Happy Boating

Well, I took the part list and changed just the parts with different numbers between the 4hp and 6hp (except the carburetor it self). In the user manual of the MFSxB is stated that the 6hp is default delivered with a 7.8x8 prop. The 4hp is with the 7" pitch.

The RPM is slightly above the 5400 rpm and the manual specifies 5000 - 6000 rpm.

After the drilling you can polish the venturi with your Dremel.

 

[pvanv]

Re: Carb modification 4hp to 6hp Tohatsu

I just bought the main nozzle (3AU032520) Main jet (3AU044680) and Pilot jet (3R1032540). I'm not sure if you need the last one but it's only $7.32. The total of all these nozzles are $ 24.38.

If you put these in the carburetor of the 4hp it won't work because of the small venturi. By removing the fuel valve (the choke valve is not easy removable) you can get to the venturi which I have increased with a table drill to 15mm (14mm will do). Everything works very smooth now.

Wow.

You have over $24 in parts, and over an hour of labor (with a costly drill press and fixturing... not to mention extreme dexterity with a dremel), so your net cost is significantly over that of a new 6A2 carb, which will idle better than a 6B carb anyway (due to meeting the earlier EPA standards), and still... the overall throat size is too small, and the OD of the venturi is a bit small, so, while having a higher output than the original setup, you are not achieving the full 6 hp.

Interesting compromise. Impressive speed results.

I suppose if your name was MacGyver, and you were living in a mud hut, that would be one option... you could even port and polish the intake and exhaust passages... but for the rest of us in civilized lands, it is cheaper and more expedient to install the bigger carb.

 

[Sea Rider]

Re: Carb modification 4hp to 6hp Tohatsu

In the user manual of the MFSxB is stated that the 6hp is default delivered with a 7.8x8 prop. The 4hp is with the 7" pitch. The RPM is slightly above the 5400 rpm and the manual specifies 5000 - 6000 rpm.

Yep, had my doubt if by changing jets and widening venturi would achieve a perfect matched 6 HP carb, there are some hidden tech facts that most mortals don't know a thing about, will include myself. Always recommend a full carb swap if updating a same detuned engine, whenever it's possible.

If with 8 pitch, lightly loaded sib and you alone have achieved 5400 rpm at wot, with 7 pitch will achieve more rpm near max factory stated numbers for that engine, will drop top rpm accordingly when adding weight.

Happy Boating

 

[Grad]

Re: Carb modification 4hp to 6hp Tohatsu

Wow.

You have over $24 in parts, and over an hour of labor (with a costly drill press and fixturing... not to mention extreme dexterity with a dremel), so your net cost is significantly over that of a new 6A2 carb, which will idle better than a 6B carb anyway (due to meeting the earlier EPA standards), and still... the overall throat size is too small, and the OD of the venturi is a bit small, so, while having a higher output than the original setup, you are not achieving the full 6 hp.

Interesting compromise. Impressive speed results.

I suppose if your name was MacGyver, and you were living in a mud hut, that would be one option... you could even port and polish the intake and exhaust passages... but for the rest of us in civilized lands, it is cheaper and more expedient to install the bigger carb.

Sure, if you have to buy the tooling or if you have to take a day off from work to get it done, you can better bring it to the dealer. Or even better, buy a 6hp right away. But don't go boating afterwards because that also takes a lot of time (= money)

I think that we have a clash here between the commercial and the hobby world.

BTW it's running very smooth in idle, better than the original setup. In the original setup it was very restless.

 

[Grad]

Re: Carb modification 4hp to 6hp Tohatsu

If with 8 pitch, lightly loaded sib and you alone have achieved 5400 rpm at wot, with 7 pitch will achieve more rpm near max factory stated numbers for that engine, will drop top rpm accordingly when adding weight.

I will re install the old 7" pitch to see the difference.

I will measure the current main nozzle and try to modify the carb with the original ones to get the same results. This could be interesting for those who like to give it a try before they buy. If it didn't succeed, you always can buy that 6A2 carb for over $100.