Tohatsu 3.5hp 2 stroke very hot under side of power head

[charybdis55]

I recently renewed the impeller and gear case oil on my Tohatsu 3.5hp 2 stroke.
The engine was working perfectly but with time on my hands and the impeller hadn’t been changed in at least 5 years, I thought it prudent to renew it.

On testing the engine in a tank, I noticed smoke coming off the underside of the power head. Also, this area was extremely hot to touch. Water coming from the tell-tale was warm.

I removed the power head and thoroughly cleaned all the water ways also re-checked the little impeller key was in position. On re-testing, I removed the power head flushing screw and plenty of water was being pumped out.

With the engine on the back of my tender, I tested it for approx. 20 mins flat out and there were no problems. A steady stream of water from the tell-tale and the water was warm.
However, the area under the power head still becomes extremely hot, like it would probably take the skin of your fingers! Also the paint is discoloured and slightly charred.

Although the engine runs perfectly and doesn’t over heat, I’m a little concerned how hot this area gets. Has anybody else had experience with this small outboards? I think Tohatsu, Nissan, Mercury are all similar.

 

[Boathook]

If this engine is also similar to a yamaha malta I suspect that the water channels on the underside are blocked. If like the malta, this means removing the engine from the leg. Quite a lengthy post a few years back started by me and still going earlier this year. Working on a tablet at present so struggle with searches.

Edit bit - Yamaha Malta
Hopefully this may help as it shows the cooling channels.

 

[charybdis55]

If this engine is also similar to a yamaha malta I suspect that the water channels on the underside are blocked. If like the malta, this means removing the engine from the leg. Quite a lengthy post a few years back started by me and still going earlier this year. Working on a tablet at present so struggle with searches.

Edit bit - Yamaha Malta
Hopefully this may help as it shows the cooling channels.

I have removed the power head from the leg and all the water channels (upper and lower) are clear.

 

[bobtooke]

I'd agree, water channel in there still at least partially blocked. My little 3.3 mariner had similar and when I stripped head off to look there was corrosion in the lower channels that I just couldn't get to without getting the power head completely in bits.

 

[VicS]

I recently renewed the impeller and gear case oil on my Tohatsu 3.5hp 2 stroke.
The engine was working perfectly but with time on my hands and the impeller hadn’t been changed in at least 5 years, I thought it prudent to renew it.
On testing the engine in a tank, I noticed smoke coming off the underside of the power head. Also, this area was extremely hot to touch. Water coming from the tell-tale was warm.

I removed the power head and thoroughly cleaned all the water ways also re-checked the little impeller key was in position. On re-testing, I removed the power head flushing screw and plenty of water was being pumped out.

With the engine on the back of my tender, I tested it for approx. 20 mins flat out and there were no problems. A steady stream of water from the tell-tale and the water was warm.
However, the area under the power head still becomes extremely hot, like it would probably take the skin of your fingers! Also the paint is discoloured and slightly charred.
Although the engine runs perfectly and doesn’t over heat, I’m a little concerned how hot this area gets. Has anybody else had experience with this small outboards? I think Tohatsu, Nissan, Mercury are all similar.

It sounds as though there is insufficient water passing through the power head to cool the exhaust

You say you removed the power head and cleaned the waterways but did you also remove the cylinder head and clean the cooing channels then accessible.

.

 

[jamie N]

With the same engine, I took the head off, and found the jacket/barrel water space totally choked with salt, leaving a small and narrow restricted path through the engine. I cleared the barrel with a long screwdriver, and removed as much of the build up as possible, reassembled and found the issue was entirely resolved. The head had no observed blockage which was curious to me, but .

 

[Seastoke]

I I have just bought a tohatsu 3.5 does anyone have a digital manual for one cheers SS

 

[Kinsale373]

I I have just bought a tohatsu 3.5 does anyone have a digital manual for one cheers SS

I have a Parts List /Catalog thats very useful with part drawings and exploded assembly views. I can't upload anything on this site. Many years ago there was a gread artivle on a Tohatsu 3.5 rebuild in PBO , Ill see if I can find a copy, I just removed my Email address on Graham'sadvie, thanks Graham! Youve recived the info I had anway
Regards, Kinsale 373

 

[Graham376]

I I have just bought a tohatsu 3.5 does anyone have a digital manual for one cheers SS

PM sent

 

[Graham376]

I have a Parts List /Catalog thats very useful with part drawings and exploded assembly views. I can't upload anything on this site. If you send an email address to ascenttp@gmail.com ill send it on . Many years ago there was a gread artivle on a Tohatsu 3.5 rebuild in PBO , Ill see if I can find a copy,
Regards, Kinsale 373

Suggest you edit your post and remove email address, better to send it by PM/Conversation.

 

[anoccasionalyachtsman]

Deleted. Wrong engine, sorry.

 

[charybdis55]

It sounds as though there is insufficient water passing through the power head to cool the exhaust

You say you removed the power head and cleaned the waterways but did you also remove the cylinder head and clean the cooing channels then accessible.

.

When the power head was off the leg, with the head upside down I had descaler (Ridlyme) foaming away for a couple of hours. As best I could, I checked the jacket around the cylinder head was clear. I have just ordered a new cylinder head gasket and will now be able to remove the head.

What puzzles me is, I've been using the engine for over a year, several times per month, each trip 1/2 hour return with no apparent problems?

I had similar problem with a VP MD11C when I changed the impeller, the engine over heated. What caused this was down to a worn face plate on the raw water pump and the new impeller didn't mate into wear grooves resulting in reduced water flow.
Putting the old impeller back cured the problem. I subsequently renewed the pump and at each impeller change, faced off the plate, no more problems.

If there's no blockage in the cylinder head and it still gets too hot to touch, I guess the impeller housing is the next thing to look at.

Just out of interest, the area under side of the power head is directly where the exhaust exits from the cylinder so I expect it to get hot, but how hot: Skin of fingers or just uncomfortable?

 

[charybdis55]

PM sent

https://www.vansoutboardparts.com/i...M2.5A2-3.5A2-3.5B2 (2011) - (00221040-6).pdf

 

[Graham376]

https://www.vansoutboardparts.com/images/pdf_nissan_tohatsu/TOHATSU/2-Stroke/Parts Catalog M2.5A2-3.5A2-3.5B2 (2011) - (00221040-6).pdf

That's the parts manual, there's also a service manual.

 

[sparkie]

When the power head was off the leg, with the head upside down I had descaler (Ridlyme) foaming away for a couple of hours. As best I could, I checked the jacket around the cylinder head was clear. I have just ordered a new cylinder head gasket and will now be able to remove the head.

What puzzles me is, I've been using the engine for over a year, several times per month, each trip 1/2 hour return with no apparent problems?

I had similar problem with a VP MD11C when I changed the impeller, the engine over heated. What caused this was down to a worn face plate on the raw water pump and the new impeller didn't mate into wear grooves resulting in reduced water flow.
Putting the old impeller back cured the problem. I subsequently renewed the pump and at each impeller change, faced off the plate, no more problems.

If there's no blockage in the cylinder head and it still gets too hot to touch, I guess the impeller housing is the next thing to look at.

Just out of interest, the area under side of the power head is directly where the exhaust exits from the cylinder so I expect it to get hot, but how hot: Skin of fingers or just uncomfortable?

Had a similar issue with my little 2hp Yamaha. Very hot in that area and little by way of flow from the (rather inadequate) telltale, didn't use it for long trips so ignored it as long as I could... anyway decided at last to investigate, everything appeared to be clear until I took the head off, the water galleries were completely choked. How it had run at all for more than 5 mins amazed me. Anyway, complete clear out and new head gasket (with some invaluable help from other readers) , and all now well, except for a slight water leak from the headjoint caused by a little corrosion of the mating faces.

Sure you'll find that that is your problem.

 

[charybdis55]

Removed the cylinder head and the water jacket was approx 80% clear. Anyway, thoroughly cleaned everywhere, repaired the sheared cylinder head bolt which i managed to shear removing the head and ran the engine up.

The area around the lower power head still gets hot, you wouldn't want to touch it when the engine was at full chat but perhaps not as hot as before I removed the head. It does cool down quickly when the revs are lowered to tick over.

The engine powers my tender without any problems so I'm inclined to leave it there.

I'm still interested to know if anyone else has one of these little 2 stroke outboards, if they can touch the area under the power head when the engine is working hard?

Had a similar issue with my little 2hp Yamaha. Very hot in that area and little by way of flow from the (rather inadequate) telltale, didn't use it for long trips so ignored it as long as I could... anyway decided at last to investigate, everything appeared to be clear until I took the head off, the water galleries were completely choked. How it had run at all for more than 5 mins amazed me. Anyway, complete clear out and new head gasket (with some invaluable help from other readers) , and all now well, except for a slight water leak from the headjoint caused by a little corrosion of the mating faces.

Sure you'll find that that is your problem.