Can a Clogged Carburetor Cause a Mower to Leak Gas?

How a Lawn Mower Carburetor Works

• Most lawn mowers with a four-stroke engine rely on a float-type carburetor to provide fuel to the engine. A float is hinged, floating valve that raises and lowers as fuel is introduced into the carburetor's fuel, or float, chamber. The motion of the float controls a valve needle, which allows fuel into the float chamber as the fuel level lowers, then closes once the fuel level reaches a predetermined height within the chamber. The fuel is sucked out of the float chamber and into the carburetor's mixing chamber before being pulled into the engine's combustion chamber.

Primary Causes of a Leaking Carburetor

• Most fuel leaks are caused when the float does not return to its highest position, leaving the valve needle in its open position and allowing fuel to enter continuously into the float chamber. The excess fuel will then leak out through the carburetor's inlet and air filter, as well as any vent tubes, or into the engine's combustion chamber. Float failures can be caused by a puncture in the float, reducing the float's bouyancy, or by a physical obstruction or binding that prevents the float from moving. However, a clog can also cause a similar problem; it forms between the valve needle and its seat. This prevents the valve needle from seating in its fully-closed position, regardless of float operation. Additionally, the valve needle itself may have worn away to create in imperfect seal that will continue to weaken over time.

Secondary Causes

• Fuel leaks are not restricted only to the carburetor's float and valve needle. The gasket that seals the float chamber to the carburetor body can deteriorate and tear, allowing fuel to weep out from the seam. Aging fuel hoses can also form leaks as their rubber shells begin to harden and crack over time. This also weakens the seal between the fuel hose and the carburetor fuel inlet. The aging process can also affect the lawn mower's priming bulb, if equipped. The soft plastic bulb will become increasingly brittle and may even crack, especially near the base of the bulb.

Repairing a Leaking Carburetor

• Start with the easiest solutions first. Check the fuel hose and priming bulb, if any, for signs of hardening or cracking. Replace either part if they are damaged. Take the carburetor off of the lawn mower and remove the float chamber if the fuel hose and priming bulb are in good working order. Look at the fuel trapped in the float chamber. Place an inline fuel filter between the fuel tank and the carburetor if the fuel is filled with debris. Inspect the float chamber gasket and replace it if it is torn or damaged. Remove the float by pushing out its pivot pin, then check the float for punctures or fluid infiltration. Replace the float as it is filled with fuel. Pull the valve needle out of its seat and check its tip for a uniform point. Replace the valve needle if its point is rounded or misshapen. Finally, clean the valve needle, float and the float pin with a high-quality carburetor cleaning solution, then reinstall everything onto the lawn mower.