Frequently Asked Questions: What would cause fuel to drip out of the mouth of my carburetor?

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FAQ: Fuel System

17. What would cause fuel to drip out of the mouth of my carburetor? - Top

The short answer is that a tiny piece of dirt is most likely stuck in the needle and seat of the float valve, preventing the valve from fully seating. However, I'd like to give you a bit more background on this important question. I used to think that the float valve in the carburetor operated like the float valve in the tank of a toilet: i.e., when water in the tank got low (as in after flushing), the valve would open until the tank filled and then close again. However, it is important realize that (unlike our toilet tanks) the needle in the carburetor float valve is always closed. What varies is the pressure that the floats exert on the needle: relatively high as the fuel level increases, and relatively low as the fuel level decreases.

This means that the pressure created by the pump (on the inlet side of the valve) is in constant equilibrium with the pressure exerted by the needle (on the carburetor side of the valve), so that, at any given fuel pump pressure between 3 and 4 PSI, a very constant level is maintained in the float chamber.

Understanding that the needle of the float valve is always being pressed against the seat is important in understanding why any tiny bit of dirt can cause so much trouble. Once a piece dirt gets caught between the needle and the seat, it will be held there and interfere with the valves ability to control the fuel level until such time as fuel is drained from the carburetor and the piece of dirt is flushed away. This is the reason for the flushing procedure in the fuel system section of our manual.

In addition to dirt causing the valves to stick open, we have seen a few cases where the brass seat of the float valve needs to be polished (smoothed out) by pressing and rotating a short pointed hardwood dowel into its small orifice. Dowels of 3/16” diameter work well. We have even had to clean up brand new seats in this fashion before they would stop leaking. - Updated: November 4, 2003

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17. What would cause fuel to drip out of the mouth of my carburetor? - Top: The short answer is that a tiny piece of dirt is most likely stuck in the needle and seat of the float valve, preventing the valve from fully seating. However, I'd like to give you a bit more background on this important question. I used to think that the float valve in the carburetor operated like the float valve in the tank of a toilet: i.e., when water in the tank got low (as in after flushing), the valve would open until the tank filled and then close again. However, it is important realize that (unlike our toilet tanks) the needle in the carburetor float valve is always closed. What varies is the pressure that the floats exert on the needle: relatively high as the fuel level increases, and relatively low as the fuel level decreases.

This means that the pressure created by the pump (on the inlet side of the valve) is in constant equilibrium with the pressure exerted by the needle (on the carburetor side of the valve), so that, at any given fuel pump pressure between 3 and 4 PSI, a very constant level is maintained in the float chamber.

Understanding that the needle of the float valve is always being pressed against the seat is important in understanding why any tiny bit of dirt can cause so much trouble. Once a piece dirt gets caught between the needle and the seat, it will be held there and interfere with the valves ability to control the fuel level until such time as fuel is drained from the carburetor and the piece of dirt is flushed away. This is the reason for the flushing procedure in the fuel system section of our manual.

In addition to dirt causing the valves to stick open, we have seen a few cases where the brass seat of the float valve needs to be polished (smoothed out) by pressing and rotating a short pointed hardwood dowel into its small orifice. Dowels of 3/16” diameter work well. We have even had to clean up brand new seats in this fashion before they would stop leaking. - Updated: November 4, 2003

[e-Mail the FAQ's] - [Search our FAQ's] - [Question Not Answered?]