Tohatsu 3.5 2T Bogs down, fixed by choke.

[Mark-1]

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.

Thanks, it is the same engine.

I've opened the drain in the carb bowl and turning on the fuel tap generates a flow of fuel out of the bowl so there is end to end fuel supply from the tank to the bowl.

I've run it with the fuel tap closed and there is a period of 'normal' running which I guess would be expected if it was over fueling.

So likely it's something in tbe carb.

I'm engaged in one final clean and then I'll consider a new carb or a service kit.

 

[Sailing steve]

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.

Fastest running will be at a optimum mixture point between too rich and too lean.

The choke restricts airflow into the engine causing the mixture to go rich which is needed on a cold engine as fuel vaporisation isn't anywhere near as efficient as when hot . Brief (couple of seconds) correct running after choke is released might be because there's still excess fuel in the induction system.

Try dripping some engine oil to make a temporary seal on any joints or gaskets the carb and engine and around the throttle butterfly spindle and see if that improves or cures the problem before you pull it all apart again.

Does it run OK flat out? If so there's another clue pointing to a small or intermittent air leak right there.

 

[Mark-1]

Further progress:

Gave the card another clean including the slider. Went to town on the main jet. No change.

Removed the fuel tank and ran it with a funnel of fuel. No change.

Undoing the drain plug on the carb bowl produces no stream of fuel. A bit comes out then it stops. (This was from the tank, I didn't try with the funnel.) So maybe the float valve or the float spring are the culprit. If I turn the carb upside down I can hear the flat moving up and down.

My thoughts are: Get an OEM carb service kit; get a new OEM carb or get a Chinese carb. (The latter two give me pretty much a whole spare carb for the future for not much more than the service kit.)

Tempted to try bending the float spring to let more fuel in and see what happens. I can't find an official source for how to set that up.

Question: There's a idle screw but it just seems to lift the slider a bit, there is no idle jet and no bypass. True?

 

[Mark-1]

Fastest running will be at a optimum mixture point between too rich and too lean.

The choke restricts airflow into the engine causing the mixture to go rich which is needed on a cold engine as fuel vaporisation isn't anywhere near as efficient as when hot . Brief (couple of seconds) correct running after choke is released might be because there's still excess fuel in the induction system.

Try dripping some engine oil to make a temporary seal on any joints or gaskets the carb and engine and around the throttle butterfly spindle and see if that improves or cures the problem before you pull it all apart again.

Does it run OK flat out? If so there's another clue pointing to a small or intermittent air leak right there.

Doesn't run flat out. It runs for seconds and dies, unless I cycle the choke. You can't "save it with revs".

So to be clear it doesn't "run better with choke" - releasing the choke gives the benefit but obvs you have to close the choke in order to release it. I should have videoed it.

I was wrong to say there was little scope for air leaks between head and carb. On second look It would be easy to fail to push it firmly against the head. This time I made sure I pushed it hard when I did up the jubilee clip.

When you say throttle butterfly I guess we're talking about the cable/brass slider at the top. I can't see how that could ever be air tight so would that be a problem? To nake sure we're talking about the same thing, here's the (close enough) workshop Manual pdf from Monty Mariner's (excellent) site:

https://montymariner.co.uk/wp-content/uploads/Mariner_3.3/Section-3-FUEL-SYSTEM-AND-CARBURETION.pdf

Thanks everyone for the help.

 

[billskip]

I have known by experience the float can leak and partially fill causing incorrect level.also the little needle valve that let's fuel in and the float closes it can be stuck/contaminated....there is sometimes a mark in the chamber for the level..but as a guide the top plate level the float arm level is the fuel off.

 

[Sailing steve]

When you say throttle butterfly I guess we're talking about the cable/brass slider at the top.

https://montymariner.co.uk/wp-content/uploads/Mariner_3.3/Section-3-FUEL-SYSTEM-AND-CARBURETION.pdf

Thanks everyone for the help.

Nope my fault, I assumed you were talking about a fixed jet carb.

That's a sliding/moving jet type which doesn't have a butterfly so you can rule air leaks there out

 

[Mark-1]

So it's running fine now, but a bit of a journey.

After the second clean failed I bought an OEM carb kit. Immediately regretted that because, it didn't contain jets, one of the parts didn't match, delicate parts came unprotected in a flimsy envelope and contained stuff I wasn't suspicious of. Moreover it was £50 whereas a new OEM carb is only ~£100 and if I'd bought that I've have a 100% known quantity carb + a spare.

Anyway I couldn't be bothered to return it for the swap so I fitted the bits I was suspcious of plus anything else that was easy to pop in. Specifically the float valve and seat and float 'holder'. The float height with the new kit was noticably different. (I had considered just bending the "float holder" and seeing what happened but couldn't find the corret height so decided to trust the service kit)

I tried it in the bucket and that was deeply dissapointing, the symptoms seemed unchanged. After it had warmed up a bit it would run without choke but didn't seem 'right'.

Decided I needed to try it on a real test spin with the dinghy to get it properly warned through. That was better, it ran without choke but still didn't seem right. It got bogged down increasing revs, it would stall increasing revs and the tickover was rough.

At that point I decided to do what I should have done all along. I drove to the garage to get fresh fuel. (Forgetting to pay i the process that that's another story.)

So the second test run was conducted when the engine had been run recently and sitting the sun for 40 minutes + fresh fuel. Again it was a little uninspiring at first but pretty soon it was running as normal.

Conclusions?

-I think the main problem was the float height, perhaps confounded later by me confusing the "symptoms" of a cold engine with the actual problem. I also wonder to what extent you can imagine a tickover is lumpy and the pickup isn't sharp enough, when it's actually fine.
- Float holders bend with time. In the Tohatsu 3.5 2T they bend to decrease fuel in the bowl and cause fuel starvation, in the Mariner 5hp 2T they tend to increase fuel in the bowl and cause flooding.
- I should have tried fresh fuel and bent the float holder a bit just to see before buying the carb kit. The only reason I didn't was I couldn't be bothered to drive to the garage and I figured if I was going to take the carb off again I might as well fit the service kit. But really if I wanted to be lazy the lazy option would be buy a new carb, which would have solved the problem quickest and given me a complete spare carb.
- Fresh fuel may be a red herring here. It was visibly perfect, smelt right, high octane E5 and only two months old which isn't ludicrous.
- The tank on these engines is low and gravity fed. The fuel pipe travels upwards at one point with low fuel the slightest backward engine angle starves the engine of fuel, IME. (Irrelevant to my issue but perhaps a confounder, for others!)
- Tohatsu 3.5HP 2Ts are awesome engines. You couldn't wish for a simpler engine. Until electric engines become more practical (and cheaper!) I'm committing to these. I keep looking for cheap non runners for spares but it looks like I'm not the only one with that idea and the prices are high.

I know these conclusion will be obvious to most other people, but it's cathartic for me to write them down.

Thanks for the help everyone.

 

[RunAgroundHard]

... get fresh fuel ... + fresh fuel. Again it was a little uninspiring at first but pretty soon it was running as normal. ...

I had the same engine serviced at Ardoran Marine. If they mentioned it once, they mentioned it a thousand times, today's petrol (UK) is no use in two strokes after about a month and promotes gumming and varnishing.

I now use an Alkaylate fuel with 2 x stroke oil added because it lasts for long intervals. However, for a long time I have been using Briggs and Stratton Fuel Fit in my fuel mix for various 2 x stroke garden tools and that keeps the fuel working over a long time, but I have not done a before and after comparison. I would add B&G Fuel Fit to your can of petrol.

What the sales blurb says: -

Keeps fuel fresh for up to 3 years
Fuel Fit not only keeps the carburettor clean for easy starting but now also protects against the corrosive effects of ethanol
Detergent ingredients to prevent gum and varnish build-up on engine parts
Metal de-activators which stop chemical reactions caused by dissolved metals in fuel
Corrosion inhibitor that forms a protective barrier on metal parts against harmful effects of ethanol-water mix

Fuel Fit https://www.amazon.co.uk/Briggs-Str...hvtargid=pla-2281435177898&psc=1&gad_source=1
Ardoran Marine Oban Marina, Oban Self Catering Holiday Chalets, Moorings, Honda Outboards, Suzuki Outboards, accommodation Oban, Argyll, Scotland - Ardoran Marine

 

[Mark-1]

I had the same engine serviced at Ardoran Marine. If they mentioned it once, they mentioned it a thousand times, today's petrol (UK) is no use in two strokes after about a month and promotes gumming and varnishing.

It's real shame that Esso stopped selling Zero Ethanol at the pumps 18 months or so ago. (For us in the South.)

My usage is too high for Aspen etc, but it's high enough I can keep it reasonably fresh. I'll never be able to keep it all to under a month, though.

 

[Boathook]

Add a fuel enzyme. I use startron? About £23 for 250ml. You don't require much so it lasts a long time.