Suzuki 2 Strokes

Forgive my lack of knowledge of carburettors. I have been doing a lot of reading about mikuni carbs and I have not found a diagram matching what I am seeing on my own 1975 suzuki a100 carb. No one seems to be able to tell me the function of this brass tube and air jet combo. (note, usually there is a brass air jet screwed into that hole - just had removed it for the photo. I believe it is size 1.6)
As far as I can tell, this air jet connects to this tube that terminates below the fuel level. I've cleaned and air blasted them out a lot to try to see if it is just plugged, but I am almost sure that they do not go anywhere else.


It is not for the enrichment circuit because that air passage is in the throat of the carb (red circle in below image) and has its own little tube going in the bowl. It is not the main air passage because that is at the throat of the carb and goes directly to the primary type needle jet (white arrow). Yellow circle is the pilot air screw. It is is not the pilot air passage as that is a fixed sized hole on the side of the carb here that goes to the pilot air screw: I dont think it is a vent as the carb has 2 other vents directly to the fuel bowl.


So what does it do? Could it just be left over from some other bike that uses the same carb design? If so why did they even bother putting an air jet in there?

Thanks for your help!

Carbs use the pressure drop between the float bowl and air passages to push the fuel through the various jets.
Early T500s use a different type of float bowl venting that later models.
That tube on your A100 carb looks to be another variation on float bowl venting.

The T500 is a piston port design with the outside of the carb exposed to outside air pressure.
The A100 is a rotary disc valve engine with the entire carb inside the “filtered air chamber”. So getting the right pressure drop across those jets probably needs that tube.

Thank you. Yes, I suppose that could be it. It does also have 2 vents directly to the float bowl that vent to the atmosphere outside the carb so i thought that might have just equalized it out anyway.

Engineering was very different back then than it is now.

They had problems with pilot jetting on the T350. They solved it by adding a second pilot jet — the T350 has two pilot jets in series. The second jet has a pretty big flow hole. First time I took one of those carbs apart, I wondered what it was doing there.

Some years ago I bought a T350 project bike that didn’t have those second pilot jets and its low speed performance was rather quirky. Installed those jets & they had a much bigger impact than I would have guessed.

Similarly, Suzuki may have added that vent to solve some performance issue.