Tohatsu 3.5 2T Bogs down, fixed by choke.

[Mark-1]

My 3.5hp tohatsu outboard won't tick over for long and when running it bogs down and stops. However, closing the choke and then opening it again has it running as normal for a while before it bogs down again. So it can be driven along oscillating the choke.

Full choke kills it, as I'd expect when it's warm, but, as I say, the application and release of choke gives it a burst of life.

I've always thought that choke on these engines just closes of the choke rather than allowing extra fuel in but my symptoms suggest extra fuel is allowed in on full choke and that fuel is keeping it running.

Fuel is E5 out of a can (just under two months old), the OB has been stored empty of fuel. I haven't checked the plug.

Anyone guess what's going on? I'm pretty sure a carb clean will fix it, I'm just curious specifically why it's misbehaving in this way.

Fortunately, cleaning carbs is absolutely my favorite way to enjoy myself so tonight is going to the best NYE ever.

 

[TSB240]

The choke just increases your fuel air ratio if cycled.
Full choke will likely stop any engine from running.
I suspect your main jet is blocked.
If you remove it you may think it is visually clear until you have passed a strand of copper wire through it and see the full diameter after removing the gum of modern bio fuels stuck to its sides.
Carb cleaners and ultrasonic baths take longer to do the job.

 

[KevinV]

Fortunately, cleaning carbs is absolutely my favorite way to enjoy myself so tonight is going to the best NYE ever.

 

[Mark-1]

Well that's interesting.

Took it apart. It all looked pretty clean, but I gave it a thorough clean. Compressed Air/Carb Cleaner/Copper wire. The main jet is deffo not the culprit. Blew though every hole I could see with the compressor.

I didn't clean the slider, but it was working freely.

I tried a new spark plug.

I could hear petrol running down the pipe when I turned the tap on so the tap is working and the pipe is clear(ish).

The float mechanism looked good, nothing obviously untoward.

No difference. At any revs from tickover upwards it dies but I can catch it by putting full choke (which starts to kill it) and letting the choke back off again. That revives it for a short period to the point where it will run indefinitely if the choke is cycled frequently.

I'll give it another clean but in the meantime what might I have missed?

It was working last time it was used. I can't see what would have changed. It was stored empty of petrol, could the fuel tap have dried out?

I didn't see a fuel filter, but if the fuel line is clogged or the tap blocked wouldn't the symptoms be different? Wouldn't it die off at higher revs and tickover fine with the limited fuel getting through?

TIA.

EDIT: Chinese carbs are available for £15-£35. Tempted to buy one to confirm the problem is the carb.

 

[Keith 66]

Blocked main or pilot jets. You aint cleaned it enough!

 

[TSB240]

How old is your fuel?

 

[Sailing steve]

Small/intermittent air leak between the carburettor and head?

 

[Mark-1]

How old is your fuel?

Two months.

2 litres of It was stored in a 12ltr tank so so space for the good stuff to evaporate and water to be absorbed. It was high octane E5.

It's a pain, but I'm wondering if I should get hold of some totally fresh fuel before cleaning the carb again.

 

[billskip]

Small/intermittent air leak between the carburettor and head?

This ^^^is what I would be suspicious about after cleaning the carb.

 

[Mark-1]

Small/intermittent air leak between the carburettor and head?

Wouldn't that make it run lean and therefore fast? And choke itself doesn't help, IYSWIM, the release of the choke makes it briefly run normally.

Good thought though, I'll have a really good look next time it's off.

 

[Mark-1]

This ^^^is what I would be suspicious about after cleaning the carb.

It's a glorified jubilee clip, not a gasket. I put it back tight. I can't really see how it wouldn't have changed when I took the carb off, even if it wasn't fixed. I'll have a really good look when I take it off next. I don't really dare clamp it much harder for fear of shearing something.

 

[billskip]

I don't know your specific carb, but many have a small (rubber?) diaphragm which can be faulty. Close inspection needed if fitted as they crack and leak.

 

[Mark-1]

On reflection it feels like fuel starvation.

Maybe holding the choke is reducing the fuel for a few seconds "saving up" a bit more to be available so it runs fine for a few seconds until that's consumed.

 

[Mark-1]

I don't know your specific carb, but many have a small (rubber?) diaphragm which can be faulty. Close inspection needed if fitted as they crack and leak.

Thanks. Is the diaphragm an elementary fuel pump? If so this doesn't have one. Gravity feeds the fuel straight in.

In fact, it can't have one, I've have the carb apart and there isn't one. On my Mercury 5hp it's a very obvious rectangle on the outside of the carb.

 

[Boathook]

I still think it is the main jet. It needs taking out and cleaning with a toothbrush bristle or similar. Use a magnifying glass to make sure it is clean.

 

[billskip]

Thanks. Is the diaphragm an elementary fuel pump? If so this doesn't have one. Gravity feeds the fuel straight in.

In fact, it can't have one, I've have the carb apart and there isn't one. On my Mercury 5hp it's a very obvious rectangle on the outside of the carb.

 

[Mark-1]

I still think it is the main jet. It needs taking out and cleaning with a toothbrush bristle or similar. Use a magnifying glass to make sure it is clean.

I'll certainly do that. I'm a little skeptical though, it's quite a large jet and visibly perfectly round. It's had wire, carb cleaner and compressed air through both bits. Worst case is it's got some varnish but would being (say) 2% smaller make that much difference especially at low revs?

 

[Boathook]

I'll certainly do that. I'm a little skeptical though, it's quite a large jet and visibly perfectly round. It's had wire, carb cleaner and compressed air through both bits. Worst case is it's got some varnish but would being (say) 2% smaller make that much difference especially at low revs?

My malta played up and I cleaned the carb a few times. The last cleaning was more thorough and the jet did look rounder. Even a very small obstruction would cause a problem. I would also double check the slow running jet as well.

 

[Mark-1]

My malta played up and I cleaned the carb a few times. The last cleaning was more thorough and the jet did look rounder. Even a very small obstruction would cause a problem. I would also double check the slow running jet as well.

Yeah, I'm much more suspicious of my attempt to clean the idle/bypass. There's no removable jet and I couldn't see either end let alone get wire in. It was compressed air, carb cleaner and crossed fingers.

My plan is:

Run it with the petrol cap off to eliminate the breather.
Run it with the fuel tap closed to see if it's flooding. (Unlikely)
Empty the fuel via the fuel tap to check the flow.
When empty get a look at the filter mesh and top of the fuel tap.
Try with 100% fresh fuel.

Then another thorough clean. Paying special attention to the Main jet, float valve, float, idle valve. Actually take the slider out this time.

These engines are really susceptible to fuel starvation when the engine is at the wrong angle - the carb can easily be higher than the fuel tap and these symptoms are reminiscent of that but I've been pretty careful to angle it forward so I'm certain that's not it. But that's how it feels.

 

[andrewAB]

I had similar symptoms and problem was the fuel stop cock/filter. Replaced it and ran fine for a season then blocked again. I now carry a spare.

I have a Mercury 3.3 2 stroke that I think is the same motor.