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Tohatsu 3.5 runs for 10 mins then dies

J

Japie

New member
My 2007 Tohatsu four-stroke, bought on e bay this summer, will run for only about 10 minutes before cutting out.
It needs to be left for a few minutes before it will start, then dies again after a few minutes.

The mechanic working on it is stumped, as he has replaced the coil and carburettor, cleaned out the fuel lines and replaced
the filter.
He has also reset the tappet clearances and stripped and cleaned out the cooling system.
Impeller was replaced and there is a good water flow.
Engine temperature is constant and normal.

He has run out of ideas, and I am running out of money !
Please help :(
 
Either you have restriction in the fuel system, where it slowly refills once off, or you have a thermal intermittent issue in the electrical system. You need to get the motor to fail, and then test/diagnose while it is not working. What is your actual model (not year).
 
pvanv said:
Either you have restriction in the fuel system, where it slowly refills once off, or you have a thermal intermittent issue in the electrical system. You need to get the motor to fail, and then test/diagnose while it is not working. What is your actual model (not year).
Click to expand...

Many thanks for replying.

I think the mechanic has tried all that.
Replaced the fuel lines and filter, then fitted another carburettor.
Also fitted a new coil that the manufacturer supplied free of charge.
He seems to think it may be temperature related, although now the cooling system
is normal.
Now she will run for a while, then die, but then restart on the first or
second pull.

Either it was overheated and damaged the piston or valves before I bought it, or,
as he thinks may be the case, there is a design fault and Tohatsu are not letting on.

The engine is an MFS 3.5
 
I suspect your correct model is MFS3.5A.
There is a long fuel filter screen integral with the fuel cock (hidden inside the fuel line from the tank). Has that been inspected?
What is the compression?
If a valve or piston was damaged, I would expect a compression problem.
Is the tank venting correctly? A partially-stopped-up vent might cause your symptoms.
Is the tank at least 3/4 full (so that motor tilt doesn't cause the tank to deliver air instead of fuel?
Is the oil level correct -- not a drop over full?
To my knowledge, that MFS3.5A is a solid design (the 3.5B is only a minor revision -- slightly different ignitor and flywheel; different EPA rating for the carb), and there aren't any known design flaws. We have sold many of these, with zero complaints. I even had one that was run without water -- major overheat -- until the plastic parts for the throttle melted and the motor seized; Once cooled down and new parts installed, it runs like new.
 
Many thanks Paul.

I saw the mechanic today and he thinks there may be an issue with the carb.
He found the jet had been drilled to make it much bigger.
After replacing it there was no difference at all.
One of the first things he did was remove the fuel filter and replace it with an in line type.
Not sure what compression is, but mechanic said it was normal.
Tank full and venting properly (we even removed the filler cap to make sure) and engine in drum
is vertical.
Oil level ok.

I am now regretting I bought the motor, but that's what can happen
on e bay :(

Regards,
Keith ( Bath UK)
 
A new carb had a drilled-out jet? Doubtful.
Did he replace the jet, or the entire carb? Regardless, the drilled carb is junque. Likely needed other (cleaning) fix, and someone decided to mess up the jet in an attempt to remedy the problem. That's seldom successful.
An in-line filter (depending on type) may present an issue, due to resistance to flow. Best to get rid of that. Temporarily running without any filter won't be an issue if you have good gas.
Compression is the PSI present in the cylinder when the 4-stroke comes on to the compression cycle. If that is good, the valves and piston are OK. Seriously, you did not know that? That is so basic, that your skill level is below what is requisite to troubleshoot and repair [any] outboard. That's not a knock, but we must understand our limitations.
There is a simple, albeit elusive, correction that will fix your issue. It is a ridiculously simple motor.
If swapping parts out in a guess (rather than diagnosing), you MUST use known-good parts. Apparently that was not the case with the carb.
Will the motor quit if it is running at higher RPM (under load of course), or is it only happening near idle?
 
Hi again Paul,
Below the latest from my mechanic. A lot of this I don't understand.

" Hi Keith
Checked the spark and the resistance of the coil before and after it ran .
Spark is present both times.
The test readings are within specification of the tohatsu service manual.
The compression is fine also
The carburettor has been ultrasonically cleaned and has had new components fitted it runs better with a larger main fuel jet fitted but it is too big to provide midrange power it floods the engine. The motor ran for more like 15 minutes and then stopped with this setup.
The engine stops because it's running lean caused by the residual heat in the block after the initial warm up by the fuel supplied by the carburettor. It's heat and fuel related but as to what it is and why it happening is where I'm stuck . I just do not want to lose hours and hours changing thing's it's not financially viable.
We will have a few more of these in over the winter so I will try and get somewhere with it.
The model and components including the carburettor fuel supply and ignition system were changed totally a year later from your model so they obviously had issues with this engine. As I'm sure you are aware we have another motor the same model year at the workshop that also had fuel issues caused by the carburettor.
The customer got fed up with it in the end a bought another motor which he has had serviced by us for a few years now without issues.
The early fourstroke motors were all rushed to market and to be honest are now into there second and third generation. The only reliable ones we have had are the early 4-6 hp tohatsu / mercury motors. Everything else has been pretty poor in all honesty including the Yamaha.
Stick with the 2 stroke for a while I guarantee it will last you for many more years than any of the four strokes ever will they are a proper marine engine and far superior in my opinion.
All I can say is it was for sale in eBay and you purchased it cheap. Perhaps this is why it was for sale in the first place .
As to the reply on the forum about valve clearances not affecting how a motor runs this is total and absolute rubbish. Metal expands when it's hot if the tolerance between the valve and the rocker arm is too small the valve stays open and you either get no compression or you end up with valves in your piston on certain models trust me I have seen it on some engine valve systems that actually open into the piston crown. the manufacturers state this to be checked and give an adjustment figure in there literature if it's not necessary.


Leave it with me . I want to sort this. I like a challenge.
Duncan "









​
 
Keith,

It sounds like your mechanic is in your corner. There is still a possibility that you have a carb problem. Once he has some known-good bits in the shop, he will be able to swap parts to find the precise issue.

The A series carbs actually run better, as they are designed to a looser emissions standard than the B carburetors. I'm not aware of any issues with the A series ignitors, but all of the Tohatsu ignitors seem to work very reliably. Yes, the 4/5/6 is an almost-indestructible design, but it is a completely different animal. The 3.5 has been reliable for us in the States... They are a little loud mechanically; some of that noise is due to the slinger (splash) oiling system. All of the emissions-rated carbs are very touchy, and must be impeccably clean to work well.

Please keep up updated as the repair progresses.
 
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