Fuel pumps on carbureted systems can be electrical or mechanical.
The mechanical fuel pump for the carburetor system is usually mounted on the cylinder head or the engine block.
It has a flexible diaphragm. That’s a flexible piece of neoprene rubber separating 2 chambers. This diaphragm is operated by an eccentric on the camshaft. It rotates, making the rocker arm move. This movement is transferred to the diaphragm, pulling it down. That draws fuel into the pumping chamber, above the diaphragm. The diaphragm spring moves the diaphragm up, and this forces fuel from the pumping chamber, out of the pump and into the carburetor.
When the engine needs more fuel, the diaphragm moves through a long stroke to pump a lot of fuel.
When less fuel is needed, pressure builds up in the fuel line to the carburetor, and in the pumping chamber above the diaphragm. The diaphragm spring can’t push the diaphragm up so far, and the pumping stroke is reduced.
Some pumps also have a return line to send excess fuel back to the tank. As the fuel circulates, it cools the fuel pump and lines, and reduces the chance of vapour locks.
