Tohatsu 4 to 6 conversion

sshow bob

Super Anarchist
2,350
296
Maine
There are a bunch of threads about annoying Tohatsu dinghy engines, and dealing with them, and also some threads about converting them from 4 to 6 hp.   I just fixed the bad behavior of mine by converting it from 4 to 6, so I thought I would write a post that someone else might find in the future when they were looking.

Short version:  do this.  Its easy, the engine runs like new, and the increased power is good.

Longer version:

We had a six year old Tohatsu 4hp on a North Atlantic Inflatables aluminum RIB. 10 feet long.  Weighs 125 pounds.  We chose aluminum because I run it up on rocky Maine shorelines several times a day to walk the dog.  With the 4hp it was displacement only with two, and would try to plane with one person, if that person went all the forward, but mostly made a lot of noise without much go.  With the engine converted to 6hp, it will plane with two (360 pounds together) and run at about 8 knots.  With just me it maintains 11 knots. 

The conversion consisted of the carb only.  I haven't re-propped it because I don't really understand props yet.  The conversion took five minutes, after I fished the engine back out of the water and dealt with drying it out.  Thank you imaginary voice that reminded me to tie a line to it.  The conversion requires the removal of two screws, and a pair of pliers to deal with the fuel line keepers.

The carb was delivered overnight by Defender.  Cost with a fuel filter and new carb gaskets was $105. The engine starts first pull again of course.

If you're reading this because you were searching for information on this conversion and wondering whether to do it: yes.  You should do it.  It does seem to drink more than half again the fuel for only half again the power.  Remote tank will go in next. 

YMMV.  Do not taunt happy fun ball.  Past gains do not guarantee future performance. Do not taunt happy fun ball, etc.

 
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sshow bob

Super Anarchist
2,350
296
Maine
I feel as though certain interesting details are missing from this story.
Only a stupid detail.  I was in the dinghy when I went to lift the engine onto the dock.  As I passed the powerhead over the tube, the boat moved away from the dock, and I couldn't hold the engine. Just a poor appreciation of physics.  No long term harm apparent at this point.

 

TwoLegged

Super Anarchist
5,665
2,081
Only a stupid detail.  I was in the dinghy when I went to lift the engine onto the dock.  As I passed the powerhead over the tube, the boat moved away from the dock, and I couldn't hold the engine. Just a poor appreciation of physics.  No long term harm apparent at this point.
That's annoying, esp when hindsight kicks in.  If only I had ... [fill in your own self-recrimination]  :(

But on the upside, you can now boast that your dinghy has that rare beast: not an outboard motor like everyone else, but an overboard motor.  And it's faster too!

 

penumbra

Member
76
25
WLIS (ish)
There are a bunch of threads about annoying Tohatsu dinghy engines, and dealing with them, and also some threads about converting them from 4 to 6 hp.   I just fixed the bad behavior of mine by converting it from 4 to 6, so I thought I would write a post that someone else might find in the future when they were looking.

Short version:  do this.  Its easy, the engine runs like new, and the increased power is good.

Longer version:

We had a six year old Tohatsu 4hp on a North Atlantic Inflatables aluminum RIB. 10 feet long.  Weighs 125 pounds.  We chose aluminum because I run it up on rocky Maine shorelines several times a day to walk the dog.  With the 4hp it was displacement only with two, and would try to plane with one person, if that person went all the forward, but mostly made a lot of noise without much go.  With the engine converted to 6hp, it will plane with two (360 pounds together) and run at about 8 knots.  With just me it maintains 11 knots. 

The conversion consisted of the carb only.  I haven't re-propped it because I don't really understand props yet.  The conversion took five minutes, after I fished the engine back out of the water and dealt with drying it out.  Thank you imaginary voice that reminded me to tie a line to it.  The conversion requires the removal of two screws, and a pair of pliers to deal with the fuel line keepers.

The carb was delivered overnight by Defender.  Cost with a fuel filter and new carb gaskets was $105. The engine starts first pull again of course.

If you're reading this because you were searching for information on this conversion and wondering whether to do it: yes.  You should do it.  It does seem to drink more than half again the fuel for only half again the power.  Remote tank will go in next. 

YMMV.  Do not taunt happy fun ball.  Past gains do not guarantee future performance. Do not taunt happy fun ball, etc.
I've been messing with my Tohatsu's for a bit with mixed results. In the past, I had the Tohatsu 3.5 2 stroke and just switching to a metal prop from the plastic adding 10-15% of speed - similar setup with an AB 9 Al RIB. I think the flex in the prop was enough to limit it. But, that was enough for a big guy and his dog to plane.

I now have the tohatsu/merc 3.5 4 stroke and went up to the carb/prop from a 5 (ReadySetBoat has a kit). The carb change was garbage. Drank fuel, had a lot of problems with leaky gaskets (turns out an off-brand gasket is much easier to align) and the idle was so annoyingly high that shifts into gear were invariably met with disdain by the wife. If anyone wants that carb, I'm happy to send it to them. I also tried to do an external fuel tank, but the fuel pump doesn't have enough lift.

I think the 2 stroke was better for my situation - higher revs meant it would plane with a light (<300 lb) load and it only weighed 30lbs. So it goes.

If your 4 has a plastic prop, I would definitely swap that out. 

 

savoir

Super Anarchist
4,907
195
I've been messing with my Tohatsu's for a bit with mixed results. In the past, I had the Tohatsu 3.5 2 stroke and just switching to a metal prop from the plastic adding 10-15% of speed - similar setup with an AB 9 Al RIB. I think the flex in the prop was enough to limit it. But, that was enough for a big guy and his dog to plane.

I now have the tohatsu/merc 3.5 4 stroke and went up to the carb/prop from a 5 (ReadySetBoat has a kit). The carb change was garbage. Drank fuel, had a lot of problems with leaky gaskets (turns out an off-brand gasket is much easier to align) and the idle was so annoyingly high that shifts into gear were invariably met with disdain by the wife. If anyone wants that carb, I'm happy to send it to them. I also tried to do an external fuel tank, but the fuel pump doesn't have enough lift.

I think the 2 stroke was better for my situation - higher revs meant it would plane with a light (<300 lb) load and it only weighed 30lbs. So it goes.

If your 4 has a plastic prop, I would definitely swap that out. 


I couldn't recommend the Tohatsu 3.5 to anyone because the carburettor is so small that it clogs super easily.  After cleaning, mine would be clogged again within 3 days.  The basic problem was that it couldn't handle heat making it useless in the tropics.  I traded up to the 4 hp which is a totally different motor and carburettor.  In 6 years all I have done is change the oil and spark plug.

 
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