Checking the Carburetor Float Valve

Last updated:

Back to the motorcycle page.

Inside the carburetor bowls, there's a mechanism that works similar to the float in a toilet. When the bowls are empty of fuel, the float opens the valve and allows fuel to enter the bowls from the gas line. When it reaches a certain level, the valve closes, preventing more fuel from entering. These floats must be set at a specific height to ensure the carburetor functions properly.

My bike has four BS32SS Mikuni carburetors. They are of the CV (constant velocity) type. The float level specification is 21.4 mm to 23.4 mm, as measured without the gasket. Setting the float height is beyond the scope of this document (for now).

After setting the height and reassembling the carburetors, I wanted a way to double-check that these valves and floats are working properly. If they're not, either there will be no fuel in the carb bowls or my engine could be flooded with fuel. My idea is to attach clear tubing to the drain screws and loop the tubing upwards. I'll then feed the carburetors with fuel and check the bowl level by observing the level of fuel in the tubing.

This project cost me a total of about $7, not including new OEM carb drain screws (which are about $5 each).

I drilled a 1/16 inch hole through the drain screws on my carburetors. I recommend not doing this by hand, but using a clamp/drill press instead.

Holes drilled in drain screws

I went to a local hardware store and purchased Gorilla Glue and 6 feet of vinyl tubing with a inside diameter of 3/8 inch. Six feet gives you 1.5 feet per carb. The 3/8 inch tubing fits over the screws tightly, so I felt the size was perfect. I also used Gorilla Glue because it expands when curing, it's waterproof, and it's compatible with vinyl and metal.


I lightly moistened both surfaces (Gorilla Glue is cured with moisture), coated the tubing and screw head with a small amount of Gorilla Glue (it expands, remember?), and finally put the tubing over the screw head. It was a pretty tight fit, so I had to use some force to get the screw head in entirely.

Glued tubing and screw

I did this four times for each of the four carburetor bowls. I used a clamp on the first one, however I noticed that it seemed to be unnecessary both because the glue expands and because the tube fits tightly. Therefore, I didn't clamp the other three assembles. In the image below, you can see the glue curing.

Glue curing

Without the clamp:

Glue curing without clamp
View of the hole

I'll update after I test this out. I strongly recommend on flushing this out after using it, as I'm not sure how gasoline will react with the glue and the vinyl tubing. I expect it would not hold up in the long-term.

Hose looped up

Back to the motorcycle page.