Tohatsu 20D Leaking Gas from Overflow

[Breeto]

I bought a used Tohatsu MFS 20D (2015 year model I think) that is leaking fuel from the overflow tube. It doesn't leak at idle, but once it's been opened up a bit it starts to leak steadily. With the help of this forum and other resources on the web, I think I *might* have narrowed it down to a missing gasket in the diaphragm assembly. It's part number 6-38 in figure 6 of this manual...

https://tohatsuoutboardparts.com/Pa... MFS9.9D-15D-20D (2015) - (002-21054-0AE).pdf

Here's my main question though. I have a pretty good understanding of the fundamentals of how a carburetor works, but after studying the carburetor and searching the Internet at length, I still don't understand how this plunger/diaphragm assembly works. I *think* it injects additional fuel into the intake port when at throttle, but the ports inside the assembly are hard to follow.

Can anyone explain its function or provide a link that does? I've searched extensively, but none of the diagrams or videos I've found look similar to my carburetor.

Thanks!

 

[pvanv]

What overflow tube?

View attachment Interactive Parts Catalogs.pdf

 

[pvanv]

The plunger is an accelerator pump that richens the mix when opening the throttle. Once you stop opening, it stops pumping. That is to reduce any stumble when accelerating. If fuel is spilling out the top vent, you are flooding. Probably the needle is not seating.

 

[Breeto]

Sorry, it looks like the correct term is "breather hose", not "overflow tube." It's part 5-35 in the link that I posted above. It looks like it's fed by two breather outlets, one on the top of the carburetor and one on the bottom. I'm not sure which one I'm leaking gas out of though. I'm not sure how I can determine that either with the carburetor already off the motor.

I've also inspected the float and needle and it looks clean, in good condition, and doesn't appear to be sticking.

Your post here, https://forums.iboats.com/forum/eng...hp-tohatsu-carb-leaks?p=10406862#post10406862, and the fact that I'm missing an O-Ring in the accelerator pump assembly is what made me suspect that was the problem and not the float valve.

By the way, thanks very much for your help, Paul!

 

[pvanv]

Sounds like some ham-fisted "mechanic" lost a part or two. Go through it very carefully, install what's missing, and you should be OK.

 

[Breeto]

Well, my theory about the missing O-ring was incorrect. I ordered the part but when I went to install it I realized it was already on the plunger. I just wasn't looking in the correct place.

I cleaned and inspected everything, put it back together, and it's still leaking. I was able to tell that it's leaking from the bottom breather hose though, not the top one.

The only other thought I have right now is to replace the plunger (https://www.boats.net/product/tohatsu/3BJ033150). I think the diaphragm attached to the plunger might be leaking. I didn't see any visible cuts or holes, but I did see some small streaks of white that I think might be part of the material underneath the outer black layer.

Any thoughts on what else I could investigate?

 

[pvanv]

Is the wee little check valve missing? How robust is the leak, er, flow? Typically a drip like that is from flooding, so re-check float height and needle seating.

 

[Breeto]

I believe all the parts are accounted for. If the check valve you're referring to is the small brass cylinder that fits in the side of the float bowl housing, then it's there too.

It's a pretty steady drip of fuel. I'd say around two to four drops of fuel per second just at idle.

I do think I need to take it apart again to do more testing of the float and needle. Last time I only cleaned it and visually inspected it, but I've since seen some videos that show how you can test it to verify it's opening and closing properly.

Is the float height even adjustable on this particular carburetor? It doesn't seem to have the little tabs that I've seen in videos of people adjusting float height.

 

[Sea Rider]

When has been disassembled, take advantage to clean the internal needle's valve seat with a Qtip, moist it im water, apply an overdose of toothpaste, introduce it on valve seat and turn it several turns while applying some pressure. Remove Qtip. wll probably turn out dirty, if so, repeat the procedure with a new Qtip till comes out clean, assemble carb and test.

All seat valves will collects over time coats of fuel impurities or whatever they are called which will stick needles in, produce fuel overflows or fuel drops spills due to needle not closing properly on valve seat. Try it nothing to lose.

If the float is all plastic, there's nothing to adjust. Be aware that the needle must be extremely sharp pointy, not rounded for the valve seat cleaning procedure to work spot on.

Happy Boating

 

[Breeto]

I replaced the plunger/diaphragm and two O-rings in the accelerator pump, and I'm happy to report that my fuel leak is now fixed! The primer bulb gets nice and firm now, and the engine is running much better. It starts on the first pull.

Thanks everyone for your help!

 

[pvanv]

Glad you got it resolved.