Common Carburetor Fuel Leaks

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One of the more common questions we get concerns either the overwhelming smell of gas coming from the carburetor or fuel leaking out of the bowl or overflow. Both conditions are typically caused by a stuck or worn float needle valve.

The float needle valve is responsible for regulating the flow of fuel into the float bowl. When the tip of the valve becomes worn or debris prevents the closing of the valve, fuel flows continuously into the bowl thus flooding the engine. Under severe conditions gas may soak the air filter or overflow out of the carburetor and onto the ground while parked.

Worn or stuck float valves are very common following prolonged storage or when gasoline has been allowed to sit inside the carburetor and begins to lacquer with age. Another common cause is the use of fuel containing any Ethanol. Many states now include anywhere from 5-15% Ethanol which can be detrimental to certain rubber parts over time.

If the fuel in your state or region contains more than 10% Ethanol you may need to replace certain soft parts more often, including the float needle valve and the accelerator pump diaphragm.

Avoid Ethanol

Never use E85 (aka FlexFuel) in a carbureted motorcycle. E85 pump fuel contains 85% + Ethanol and will begin to damage parts in the carburetor that were never designed for this type of fuel.

We always recommend replacing the float needle valve every 2-3 years or immediately following any prolonged storage where a fuel stabilizer was not used. If you used a high content Ethanol fuel be prepared to replace this part much sooner.


One major concern following a stuck/worn float valve and flooding is the resulting damage that can occur to your engine. Fuel that is allowed to overflow or flood back into the intake will wash down the walls of the cylinders. This not only increases the possibility of piston ring wear but the contaminated oil is unable to lubricate bearings or other critical parts properly. If you suspect that a bad float valve or similar condition has been allowing unburned gas to flow into the engine then we recommend an oil change while fixing the original cause.

Float Valve Types

Just a note on the types of float needle valves. There are 2 types; 3 sided and 4 sided. both work the same despite years of rumors that the 3 sided valves should not be used. Back around 1992 Harley-Davidson released a bulletin warning not to use a 3 sided valve in the newer CV carburetors. This was actually in reference to an early style 3 sided valve that had been used on the earlier Keihin butterfly carburetors. Unfortunately the motor company never put out an update when they and others began making 3 sided valves that were compatible with the CV carburetor, so rumors still circulate today about not using this style valve (yes 24 years later). The below pictured float valves are both compatible in a Harley CV carburetor.

Harley float valve styles
We sell both our own 4 sided valve along with rebuild kits from our suppliers that contain the 3 sided valve. Both will work equally however, if you still only want to use a 4 sided valve we have them available here:

ALWAYS reset your float bowl level after installing a new float valve.

For a complete list of replacement carburetor gaskets visit:

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