Tohatsu MFS3.5B idling problem

[Jamie Dundee]

Earlier this year I bought the above 10 year old outboard for my tender. I was aware it had an idling issue but suspected a manky carb from lack of use. Finally got round to stripping it down a few days ago and found that a previous owner had fitted one of the inlet manifold gaskets to the air intake side, effectively blanking the slow air jet and restricting airflow in general. Took out the jets and gave the carb a good clean with carb cleaner, fitted two new inlet manifold gaskets and a new float bowl seal. Put it all back together, started first pull, and.....it doesn’t idle Plenty of fuel getting through and seems to run better at higher revs (sea trial later today) but no joy at tickover. Any ideas from the collective?

Just to add, I fitted a new spark plug, impeller, changed gearbox and engine oil etc when I bought the engine.

 

[coopec]

Have you tried looking for the problem on other threads?

Problem With New Tohatsu 3.5

Problem With New Tohatsu 3.5

3.5hp carb problems - Marine Engine

3.5hp carb problems

Tohatsu MFS3.5B idling problem YBW - Google Search

 

[Jamie Dundee]

Have you tried looking for the problem on other threads?

Problem With New Tohatsu 3.5

Problem With New Tohatsu 3.5

3.5hp carb problems - Marine Engine

3.5hp carb problems

Tohatsu MFS3.5B idling problem YBW - Google Search

Thanks for the links, they don’t seem to replicate my problem though. Having found that someone has already been in the carb (misplaced gasket, nasty liquid gasket on float bowl) I suppose I should replace all the jets, float and needle etc but having already replaced the tank (“leaky cap“ turned into a badly cracked filler neck) I’m loathe to be throwing much more money at it. One thing I did notice yesterday was that as I ran the carb dry after turning off the fuel it did actually run quite smoothly at lower revs before it ran out, which would indicate to me that it’s over fuelling.

 

[[159032]]

Still points to idle setting. Have you used an airline to blast through the carb? That jelly gets stuck everywhere.
A soak with carb cleaner may help. As there is evidence of hamfisted work on it before (gasket in wrong place, etc) you could investigate whether the jet has been 'poked out' in an effort to clean it. It's not that easy to open the hole up, but someone may have tried hard.
It seems idling revolves around the main jet tube (no 9 in diagram) and air flow (pilot screw). Diagram for: INTAKE MANIFOLD & CARBURETOR for TOHATSU MFS3.5B (3.5 hp) outboard motor

I note in one of the links a reply from 'Paul, Certified Tohatsu TLDI Technician'.
"you must have a tach in order to adjust the idle speed."

Lord, save us from certified experts ....

 

[VicS]

Thanks for the links, they don’t seem to replicate my problem though. Having found that someone has already been in the carb (misplaced gasket, nasty liquid gasket on float bowl) I suppose I should replace all the jets, float and needle etc but having already replaced the tank (“leaky cap“ turned into a badly cracked filler neck) I’m loathe to be throwing much more money at it. One thing I did notice yesterday was that as I ran the carb dry after turning off the fuel it did actually run quite smoothly at lower revs before it ran out, which would indicate to me that it’s over fuelling.

Have you tried adjusting the idle mixture screw ? ( Initial setting is I believe 3½ turns out from gently seated then turn in and find best setting)

Have you checked the float level

Have you cleaned, inspected, tested or replaced the needle valve. Unfortunately there is no replaceable seat. If that's worn or damaged I guess you are stuffed

The idle mixture screw (#20 below) may be hidden under a plug

 

[[159032]]

Yes. The Pilot Screw is your main adjustment for idle mixture.
The base setting is to screw it in fully (soft landing! If you have to wind it in more then 2 turns to close it, it was set too rich, be sure to count) and then open it up (anti-clockwise) one and a half turns. . After that open it further to richen the idle and close it to weaken it. When it runs smoothly adjust the engine speed with the throttle stop screw.
Tohatsu recommend you make these adjustments when the engine is warm and in gear and under load. So either made fast to the jetty, or possibly in a test tank (known as a wheelie bin)

 

[C08]

My Tohatsu gave similar problems that turned out to be the float needle valve. The needle end had corroded away and the fuel level was not corect.
I would buy a new carb if the rest of the motor is ok, not expensive now for these old engines.

 

[Jamie Dundee]

Just been out for a spin, ran great (apart from idle) for 5 minutes then suddenly died. I’m thinking more and more it’s a float needle/seat problem. The whole carb has been blown through and cleaned, not much evidence of gunk tbh. I was wondering about the pilot screw but it seems that is hidden behind a brass blank. Toss up now between buying a new carb - £130 - or binning this for parts and buying a new engine. I’m leaning towards the latter at the moment.

 

[Beneteau381]

Just been out for a spin, ran great (apart from idle) for 5 minutes then suddenly died. I’m thinking more and more it’s a float needle/seat problem. The whole carb has been blown through and cleaned, not much evidence of gunk tbh. I was wondering about the pilot screw but it seems that is hidden behind a brass blank. Toss up now between buying a new carb - £130 - or binning this for parts and buying a new engine. I’m leaning towards the latter at the moment.
View attachment 92359

How much do you want for it?

 

[[159032]]

Just been out for a spin, ran great (apart from idle) for 5 minutes then suddenly died. I’m thinking more and more it’s a float needle/seat problem. The whole carb has been blown through and cleaned, not much evidence of gunk tbh. I was wondering about the pilot screw but it seems that is hidden behind a brass blank. Toss up now between buying a new carb - £130 - or binning this for parts and buying a new engine. I’m leaning towards the latter at the moment.

Lever the blank out (you have identified it right?) If it works you have saved £130 or a new engine cost.

What's to lose?

.

 

[[159032]]

Have you tried adjusting the idle mixture screw ? ( Initial setting is I believe 3½ turns out from gently seated then turn in and find best setting)

I only see reference to One and a half turns on the internet.

 

[VicS]

I only see reference to One and a half turns on the internet.

The manual for the 4 & 5 hp definitely says 3 ½ and it looks like the same carb .... I also found 2½ mentioned somewhere. 1½ doesn't seem very much but its not important its just an initial setting
Get it running , slow it down as much as possible with the idle speed screw. Try backing out the mixture screw a little at a time until its obviously a tad too rich then slowly , 1/8 the turn at a time, screw it in again ( keeping reducing the revs if it speeds up) until it falters. Then back out 1/4 turn. Somewhere in that 1/4 turn it should be possible to find the best setting. Finally readjust the idle speed to correct/ required rpm

What we need is a service manual for the 3.5 !

 

[VicS]

Just been out for a spin, ran great (apart from idle) for 5 minutes then suddenly died. I’m thinking more and more it’s a float needle/seat problem. The whole carb has been blown through and cleaned, not much evidence of gunk tbh. I was wondering about the pilot screw but it seems that is hidden behind a brass blank. Toss up now between buying a new carb - £130 - or binning this for parts and buying a new engine. I’m leaning towards the latter at the moment.

At least check the float height and inspect the needle for wear or damage
and lever the plug out to check the idle mixture setting.

If you have not already removed the plug and the mixture screw you have not cleaned the idle passages properly have you!

Still might get it going for under a tenner.

BTW is the rubber cap, #19 in the diagram I posted , in position. I dont know what it is but it would be as well to check its there

 

[Jamie Dundee]

How much do you want for it?

2 pints of lager and a packet of crisps.

Postage from the Isle of Man will be about 300 quid though...

 

[[159032]]

The manual for the 4 & 5 hp definitely says 3 ½ and it looks like the same carb .... I also found 2½ mentioned somewhere. 1½ doesn't seem very much but its not important its just an initial setting
Get it running , slow it down as much as possible with the idle speed screw. Try backing out the mixture screw a little at a time until its obviously a tad too rich then slowly , 1/8 the turn at a time, screw it in again ( keeping reducing the revs if it speeds up) until it falters. Then back out 1/4 turn. Somewhere in that 1/4 turn it should be possible to find the best setting. Finally readjust the idle speed to correct/ required rpm
What we need is a service manual for the 3.5 !

BTW is the rubber cap, #19 in the diagram I posted , in position. I dont know what it is but it would be as well to check its there

True. Once it is running you can weaken or richen it to suit.

I don't know about the rubber cap but did come across mention of it a blog in which the owner replaced it after having it loose in the float chamber. It stopped the carb tending to flood, apparently.

.

 

[Jamie Dundee]

Just had another go at it with little result. The float needle looks pretty dull but no obvious damage and moves freely. I see no way on God’s earth of levering out the brass blank over the idle screw (see below). Think I’ll speak to the guys at Cambridge outboards on Monday with a view to getting a whole new carb. Given what I’ve already spent it’s probably worth getting right.

Btw, the rubber plug next to the main jet is still in situ. Looks like there is another jet under it, as a last resort I might try removing the plug and see what happens.

 

[Beneteau381]

Just had another go at it with little result. The float needle looks pretty dull but no obvious damage and moves freely. I see no way on God’s earth of levering out the brass blank over the idle screw (see below). Think I’ll speak to the guys at Cambridge outboards on Monday with a view to getting a whole new carb. Given what I’ve already spent it’s probably worth getting right.

Btw, the rubber plug next to the main jet is still in situ. Looks like there is another jet under it, as a last resort I might try removing the plug and see what happens.
View attachment 92368

Drill a small hole in that brass plug, screw a self tapper in and use that to lever the plug out, then get the screw out, I did a chain saw last week with a similar prob, git the screw out and hacksawed a slot in it so It could be adjusted

 

[Jamie Dundee]

Drill a small hole in that brass plug, screw a self tapper in and use that to lever the plug out, then get the screw out, I did a chain saw last week with a similar prob, git the screw out and hacksawed a slot in it so It could be adjusted

I’m picking up a new motor tomorrow so this one will become a project. I’ll drill and remove the blank and see where we go from there. The engine is in decent condition and not that old so it ‘should’ work well.

 

[Jamie Dundee]

Just to put this one to bed, put a new carb on the engine, started first pull and idles perfectly. I think either someone has cocked up the float height or rodded through the needle jet/seat with a skewer.

So now I have a choice of dinghy engines. Deep joy.....?

 

[bluerm166]

Bizzarely this post coincides with carburettor work this week on my MFS 3.5A (flooding) and explains why early in the motor life the original new but faulty carb was replaced FOC by the dealer with what I took to be an 'after market' replacement i.e a KEIHIN 3GVABH.So not like the carb shown in my Tohatsu full service manual but with a different float and totally different slow jet position.So thanks very much to Mercury Rising (and I see an outfit in Moskow) for the diagram which shows me where the slow jet is tucked away on what turns out to be the 3.5B carb.
I also had a lot of difficulty starting and keeping the engine running initially after buying S/H but have since had success after treating the fuel with Star Tron additive and running the engine dry.