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