Fuel pump

Virago vacuum fuel pump

A fuel pump is used to transfer fuel from the tank to the carburetor or fuel injector. A pump is required when the fuel tank is below the fuel system and the force of gravity is insufficient to get fuel to the carburetor. Such instances include when the fuel is stored under the seat or when the tank is close to parallel with the carburetor.

A fuel pump works by a diaphragm moving and creating alternating vacuum and pressure. This drawing and pushing is regulated by check valves that only allow one-way flow. The geometry of the diaphragm plus the speed of the pulse dictate pressure and volume flow.

There are two types of fuel pumps on motorcycle engines: vacuum and electrical. Both perform the required function for the specific design of the engine. On modern motorcycles with fuel injectors, an electric fuel pump is required to create the appropriate pressure required for fuel injection.

Motorcycle manufacturers design fuel systems to have specific flow and pressure requiring specific fuel pumps. The pump is usually located between the tank and carburetor and can be tucked under the frame where the tank is mounted. Some pumps are even located inside the bottom of the fuel tank.

Vacuum pump

Engine vacuum is used to operate a vacuum fuel pump by pulsing a diaphragm as described above. Since the vacuum of the engine has a bit of a pulse, the pump can flow fuel to the carburetor through all RPM’s. Hoses and fittings transfer the vacuum of the engine to the fuel pump.

Electrical pump

Virago electric fuel pump

Instead of the engine vacuum pulsing a diaphragm, on electric pump this is accomplished by either an electric motor or a solenoid. As a motor spins, it moves an impeller that pushes fuel. A solenoid can create this effect by turning an electrical current on and off, which pulses the diaphragm up and down. An electric pump will also have a fused relay. A relay is a switch that uses a small amount of current to switch a larger amount of current. This allows the low voltage ignition switch (TCI) to electrically command on the pump.

Signs That The Fuel Pump Is Failing

Here are some common symptoms of a failing fuel pump:

• Engine will crank but not start
• Engine will have difficulty starting
• Poor performance on acceleration (stumbling)
• Engine will surge or sputter
• Poor idling
• Overall loss of engine power
• Engine will cut out
• Whining noise from the pump

If your engine is having a hard time starting or refuses to start, there are 3 things to consider: compression, spark, and fuel. If you don’t suspect poor compression and checked that the spark plugs are good, you might be looking at why there is no fuel. A common-sense method would be to verify that there is fuel in the tank. If there is and the engine still isn’t getting fuel, it could be the pump is going bad.

If you can get the motorcycle to start and notice that the idle is stumbling or that the acceleration is weak, this could due to the pump underperforming. As the pump starts to wear out, it is unable to keep up with the demand of the engine. Simply put, it’s not getting enough fuel to the carburetor. This low-flow of fuel will also cause stumbling, sputtering, or surging. In some cases, the engine will just quit running. 

A symptom you may notice that is usually a telltale is a whining noise from the pump. As the pump wears out, this creates larger tolerances between moving parts and that means more vibrations that produce a whining noise.