My Mikuni Carburetor Leaks Gas From the Vent

by Chris Gilliland

Mikuni's range of carburetors are a common sight on nearly every Japanese motorcycle ever built, ranging in form from a simple round-slide carburetor to the more-complex constant velocity carburetor. While a Mikuni carburetor is reliable and generally requires little attention beyond regular cleaning, problems can and will occur. A fuel leak, starting from the carburetor's vent tube, can be worrisome affair for many motorcyclists. In order to fix the problem, you need to understand what causes the leak in the first place.

The Basic Carburetor

Mikuni's motorcycle-specific carburetors use a float-type fuel delivery system. The float is a hinged valve that literally floats on the fuel surface within the carburetor's lower float chamber. As fuel is drawn into the carburetor jets, the fuel level decreases and lowers the float. This in turn opens the valve and admits more fuel into the float chamber, raising the float again and closing the valve. This process occurs over and over, keeping the fuel at a predetermined level at all times.

Understanding Carburetor Floats

A carburetor float is usually a plastic or brass component that is made up of two floating ends joined by central bridge that pivots within a pair of towers cast into the bottom of the carburetor. Most Mikuni floats have a tang at the center of the bridge that is used to push the fuel valve upward into the valve seat, restricting the incoming flow of fuel. In some cases, the tang is adjustable to raise or lower the fuel level for performance tuning purposes. If the tang is moved upward, the fuel level is decreased and vice-versa. It should be noted that plastic floats usually do not have this type of adjustment available to them.

How Float Height Affects Fuel Level

The carburetor's internal fuel level is directly correlated to the float height, as determined by the float's tang. If the tang is lowered too far -- bent down to an almost flush position with the central float bridge -- the float's height, and the fuel level, will increase beyond a safe limit and overfill the float chamber. The excess fuel will begin to leak out of the vent tubes built into the carburetor body in an attempt to return the fuel level to its normal limit. However, since the float height was set too high, it will continue to admit fuel into the carburetor. Additionally, the excess fuel may find its way into the engine through the carburetor jets, where it could leak past the piston and into the engine oil. The combination of fuel and oil results in a thinned mixture that reduces the oil's ability to lubricate the engine, causing premature engine wear and failure, if left unchecked.

Adjusting Float and Fuel Level

The exact method to reset your carburetor's float height and fuel level will differ between motorcycle manufacturers and individual model types. This being said, most manufacturers specify a maximum fuel level between 10 to 25 mm below the float chamber's mating surface. This is checked by placing a clear plastic tube onto the drain nozzle at the bottom of the float chamber, which is then bent up and around the side of the carburetor in a U-shape. The float chamber drain screw is then loosened to fill the tube with fuel. The amount of fuel held in the tube is representative of the internal fuel level. Next, the carburetor is removed and drained, then the float chamber is removed to access the float. The float tang must be bent either upward to lower the fuel level, or downward to raise the fuel level. Once the float has been adjusted, the carburetor is reinstalled and the fuel level check must be performed again.

Additional Causes

A vent hose fuel leak can also occur if the float valve does not seat properly in a fully closed position, allowing fuel to continue to fill the float chamber regardless of the float position. This is often caused by debris trapped between the float valve and valve seat or deformation of the float valve tip. The float valve should be inspected and cleaned, if the fuel leak continues after setting the fuel level via float height adjustments. The valve tip should appear uniformly pointed without any side-to-side deviations or blunting. If the valve is distorted or worn, both the valve and its seat should be replaced immediately.

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