How do Mikuni BS34SS Carbs vent float bowl?

[atBike22]

Hello, I am trying to understand how my carbs should be venting, as I believe it is causing an issue with fuel delivery. These carbs do not have a vent tube, but instead are supposed to vent back through a passage near the pilot air jet. Inside the carb body, I can see a passage leading from the top of float chamber, down through the bowl gasket, into a passage along the float bowl mating surface. However, I do not see where the air is supposed to go from there. The only other hole in the gasket that leads from this passage is the hole for the choke pickup tube. I do see a hole in the mating surface up on the carb body, next to the pickup tube, but it seems to be only a millimeter deep, and there is no hole in the bowl gasket for it. Is there something I am missing, or does it vent somewhere completely else? I will try to provide photos when possible.

[atBike22]

[Nessism]

Most carbs have plastic Tee, one each between the two outer carbs, that function as a vent. A piece of hose should be attached to the Tees, and run up to a safe place, above the airbox somewhere. Other model BS carbs, vent out the intake side of the carb, and into the airbox. That's why, when a float valve sticks, the airbox fills with fuel. Check your particular set, and follow the path to know what you are dealing with.

Have you checked the carb rebuild tutorial linked in my signature?

[atBike22]

Thanks for responding! I have an 85 GS450, so only two carbs, and only one fuel tee that only connects to the petcock. I do see the hole in the intake of the carbs coming from the airbox, across from the pilot air jet, but I am not sure where it should lead to in the float chamber. I don't see carb cleaner come from anywhere when I spray into the intake vent hole, so I believe the passage may be blocked. In the carb rebuild tutorial, it also shows the vent hole, but not where it leads to.

[Nessism]

I'm not specifically familiar with those carbs, but most are similar.

Did you ungang the carbs for proper cleaning? Remove the float bowl, float, float valve assembly? If not, that's how you start.

Look at the carb body, at the roof of the float chamber. There will be some sort of passage at the top, or said another way, the bottom of the carb body. Check these passages, and shoot carb cleaner though them, and see where they come out. At some point, you will realize that there is a vent to the chamber. It will either come out some nipple, or into the carb throat. That's your carb bowl vent.

[atBike22]

Hello, looking at the roof of the float chamber, I only see one passage, other than the jets. It can be somewhat seen in this tutorial on page 33, just above the pilot jet. https://gsarchive.bwringer.com/mtsac...ing_Series.pdf
It connects to the hole in the mating surface slightly left of it, where it goes "down" through the gasket into a passage along the top of the float bowl mating surface. This passage only seems to connect to where the choke pickup tube enters the float bowl. I will keep looking for another hole however.

[Nessism]

This tutorial is better... https://gsarchive.bwringer.com/mtsac...d_Tutorial.pdf

And see these holes, chase them. Find out where they come out in your carbs. This photo is from a set of GS250/300 carbs, but it should be similar to yours. Follow the path to the holy land...

Screenshot 2025-08-14 143851 by nessism, on Flickr

[atBike22]

Now that I have the carbs apart again, I can see there IS a vent nipple on each carb, just above the fuel tee. There was never any hoses connected to them, so I had assumed that they were just another plugged off passage when I rebuilt the carbs. Thank you so much for your help! I will be ordering new hoses for them today. Also, would you happen to know where the hoses should connect to? I don't see any indication of them in the manual, but the hoses are present in the Partzilla diagram.

[Nessism]

Also, would you happen to know where the hoses should connect to?

As posted earlier in this thread...

"Most carbs have plastic Tee, one each between the two outer carbs, that function as a vent. A piece of hose should be attached to the Tees, and run up to a safe place, above the airbox somewhere."

[Don R]

The vents should terminate into still air. That's usually under the seat above and behind the airbox. A friend thought it was a good idea to terminate them inside the airbox so a vacuum would help pull fuel into the bowls. I never did that.