Tohatsu 3.5 2-stroke problem. Anyone pointme in the right direction please?

Bobc

Bobc

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I have what I think is a fuel starvation problem but can't seem to find the fault.

It starts and runs fine, ticks-over fine, and will run in gear fine up to about 1/3 throttle for ever.

If I put the throttle up, it goes fine for a couple of seconds, then dies. However, if as it dies I put the throttle back towards idle, it will keep running.

So, I removed the carb, gave it a good clean with carb cleaner, and blew out all the jets and passageways (none looked blocked). I checked the tap, and there's a good stream of fuel coming from it, and checked the needle valve, which is not sticking.

As far as I can see, the only jet is the main one in the centre (which I'm guessing is the one that would most likely cause a problem on high throttle).

So I'm a bit lost as to where to look now. It's not an old engine and has not done many hours, so I suspect that it's a bit of something blocking a vital hole, but I''ve no idea where to look now.

Help! It's driving me nuts...
 
Have drained and replaced fuel with new (that was my first thought). Opened the main fuel cap to make sure, but no lack of fuel coming though the tap anyway, so can't be that.

Definitely not a spark plug problem, as it starts and runs perfectly at tickover and up to 1/3 throttle.
 
Spyro said:
Sticking float?
Click to expand...


I'm thinking that it might be a lack of fuel in the float bowl somehow. I've checked that the float has no leaks. It's one of the round ones with a hole. Doesn't attach to anything, just "floats" there as it were.
 
Sounds very much like fuel starvation. Is there a filter in the tank above the tap? Has the pipe from the tap to the carb collapsed? Have you tried running with the fuel cap off? (although you'd need the tank pretty full for that a blocked breather have an effect particularly quickly).

If the float level is too low that just might have an effect, but slow running would be tricky as well, and starting. Is the carb clamped to the manifold tightly?

Fuel filter is my favourite possibility. I've had that happen on a couple of engines.
 
Almost certainly a partial blockage due to gunge in the fuel line from the tank to the carb. While you sit the carb fills up it starts and runs at tickover but throttle up and it dies 10 seconds on.

The fuel filter in the fuel switch is the major suspect but I have had a worm of jellied fuel in the actual line. You may have to go through the cleaning routine 2 or 3 times to clear the gunge. At least I do.

NB it is still the best egg beater around.

Note to self "Always run the damn thing dry before storage"
 
Bobc said:
Have drained and replaced fuel with new (that was my first thought). Opened the main fuel cap to make sure, but no lack of fuel coming though the tap anyway, so can't be that.

Definitely not a spark plug problem, as it starts and runs perfectly at tickover and up to 1/3 throttle.
Click to expand...

I suggested trying a new plug as I have experienced similar "fuel starvation" symptoms in my days of tearing around on and off road on 2 stroke motorcycles. All was fine until the throttle was opened up then the power died, only to return as less throttle was applied. Exhaustive checks like yours showed the fuel system to be OK and a new spark plug cured the problem.
 
typhoonNige said:
Have you tried a new spark plug?
Click to expand...

Bobc said:
Definitely not a spark plug problem, as it starts and runs perfectly at tickover and up to 1/3 throttle.
Click to expand...

If there is a good flow of fuel reaching the carb and the carb is clean I would try a new plug!

Too often what appears to be a fuel/ carb problem is cured by fitting a new plug for it to be dismissed in this way
 
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Bobc said:
I have what I think is a fuel starvation problem but can't seem to find the fault.

It starts and runs fine, ticks-over fine, and will run in gear fine up to about 1/3 throttle for ever.

If I put the throttle up, it goes fine for a couple of seconds, then dies. However, if as it dies I put the throttle back towards idle, it will keep running.

So, I removed the carb, gave it a good clean with carb cleaner, and blew out all the jets and passageways (none looked blocked). I checked the tap, and there's a good stream of fuel coming from it, and checked the needle valve, which is not sticking.

As far as I can see, the only jet is the main one in the centre (which I'm guessing is the one that would most likely cause a problem on high throttle).

So I'm a bit lost as to where to look now. It's not an old engine and has not done many hours, so I suspect that it's a bit of something blocking a vital hole, but I''ve no idea where to look now.

Help! It's driving me nuts...
Click to expand...

I had the exact same symptoms with my one season old unit. After replacing fuel, checking the rubber fuel hose/tap from tank to carb.were not blocked and dismantling the carb bowl/float, jet I found that although I had made it a practice to run the motor dry there was a problem with the float valve.

It is made from a poor quality alloy and a minimal amount of corrosion had appeared on its surface so that it did not slide up and down freely so when the float dropped as fuel level fell the valve stuck causing a fuel shortage; the problem was not all that apparent but was made worse by the fact that the valve has very little weight (unlike old fashioned brass ones), and thus sticks easily.

the solution was to use a very fine emery cloth and lightly rub it down and all was then well.

BEWARE- when dismantling the carb that you do not lose the tiny s/s bar which serves as the pivot point for the 'U' shaped fork which sits atop the float, as that bar is a loose fit and easily overlooked and will fall out.

Good luck
 
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Thanks for the replies so far.

I'll take the fuel pipe off and blow it through to make sure it's clear, re-check the needle valve, and replace the spark plug for good measure, and then report back.
 
Could be loss of crankcase pressure, as the revs increase pressure is lost, throttle back and the seals 're-seal'..

Check the gasket /sealant joint between the two halves, and replace crank seals ..

good luck.
 
Having had pretty much the same symptoms on the same base engine recently, they did not clear up until, I'd emptied all the fuel, cleaned the carb, cleaned the tank, blown out the fuel line from the tank the carb.
Always carry a spare Spark plug and try that first as its a lot less hassle than the above.
The tell tale was looking into the fuel I drained into a spare 2L bottle, it had quite a few drops of black water/ muck in the bottom of it!
 
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