Some tanks are located under the carburetor, where a pump and weir system is used. This is the situation in the small engines of Briggs and Stratton.
- The fuel is pumped from the tank by a suction-operated diaphragm of the air inlet.
- It then flows into a small chamber/tank with a landfill that allows the gas to overflow into the tank once the camera is full. This ensures that the fuel level is constant, regardless of the fuel level in the main tank.
- The mesh fuel filters cover the ends of the tubes that siphon fuel from the tank to the Chamber and the Chamber/tank to the carburetor venturi, and these can be covered with dirt, restricting the flow of fuel.
- The diaphragm pump of this type of system can become worn and punctured, which causes the carburetor to absorb too much fuel. The symptoms are an engine that accelerates repeatedly up and down, produces a black ray, plus an odor of unburned gas. Alternatively, the pump can be stretched and "idle, in which case it will not be pumped at all. The only solution is to replace the diaphragm.
- In addition, single-sense reed valves (which are only fins that form part of the diaphragm) may become clogged on tank/carburetor surfaces due to rubber deposits if fuel is left in the tank during the winter. This is why it is important to dry the lawnmower before storing it during the winter, and add a fuel stabilization product like STA-BIL to the gas to keep it cool. Ethanol in fuel attracts moisture that can be harmful to carburetors, causing corrosion over time and requiring carburetor reconstruction.
