How Does a Small Engine Carburetor Float Work?

The internal combustion engine was first successfully designed by Nicolas Otto in the late 1800s, but its design--while better than earlier attempts--was still crude. A few years later, Karl Benz, a German engineer and founder of Mercedes Benz, developed the carburetor.

While engines seem to be complex, their operation is quite simple. The combustion of a fuel and air mixture inside a sealed cylinder converts stored energy into motion. Fuel and air gets introduced to the cylinder by either carburetors or injectors. Carburetors have a series of fuel flow control systems in them, and the float bowl is the first in that line.

Carburetors have a small fuel storage tank that's integrated into their design to keep a ready supply of fuel at hand. The tank's fuel level is controlled by a float, much as a float inside a toilet tank. The amount of fuel available might be only a few fluid ounces or as much as a full cup, depending on the size of the carburetor and engine.

Whether on a small or large engine, fuel must be readily available at the point it enters the engine. Fuel in a gasoline tank is routed through a fuel line to the inlet of the carburetor. Pressure, either by a pump or by gravity feed, forces fuel into the temporary storage well, called a float bowl.

Once inside the inlet to the float bowl, the proper level of fuel is maintained by the float. The float is a buoyant device that is attached to an arm. This arm acts as a lever when it rises or falls with the fuel level, and at the opposite end of that lever is a valve that opens or closes the incoming fuel from the gasoline tank.

A float bowl and float are similar to the configuration found in a toilet tank. If the lid to a toilet tank is removed, an arm with a ball (or a "post" with a sliding float) will be seen. When the toilet is flushed, the water stored in the tank is released into the toilet. The ball drops and a water valve is opened. Once that cycle is flushed, the water refills the tank through the inlet valve. When the tank is full, the float causes the valve to close.

Their operation is simple and the reliability is high, but occasionally a repair is required. Floats are made of either brass or petroleum resistant composites. If a float becomes saturated or leaks, the fuel level could get too high. With a few tools, it is a simple component to replace.

Carburetors are becoming less common in automotive applications. In small engines, such as a riding lawn mower, more efficient technology of fuel management used in cars, trucks and motorcycles (electronic fuel injection) is not practical. The low cost and simplicity of carburetors will keep them in high demand for small engine applications well into the future.