How Does A Motorcycle Fuel Pump Work?

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. 

So how does a fuel pump work? 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.

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. The location may vary, so refer to the specifications of your bike for further information.

How Does A Fuel Pump Work?

There are two types of fuel pumps on motorcycle engines: vacuum and electrical. While these are not normally interchangeable, there are ways to upgrade a vacuum pump to an electrical pump. Both perform the required function for the specific design of the engine. On modern motorcycles, an electric fuel pump is required to create the pressure required for fuel injection.

A vacuum pump works a little differently than it might seem. Engine vacuum is used to operate a 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.

An electrical pump is pretty straight forward. Instead of the engine vacuum pulsing a diaphragm, this is accomplished by either an electric motor or a solenoid. As a motor spins, it moves an impellor 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. Having a fuse in the circuit protects components against damage in the case of a circuit short to ground. A relay is a switch that uses a small amount of current to command a larger amount of current. This allows the low voltage ignition switch to command on the pump.

Signs That The Fuel Pump Is Failing

There are many reasons why a fuel pump is failing and different ways to diagnose it. 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 know 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.

How To Fix Or Replace A Fuel Pump

If you start to notice any of these symptoms, it’s time to start doing some diagnosing. Pinpointing a bad fuel pump can be an immersive process, so make sure to plan to have some extra time. If that isn’t an option, call around to find a competent mechanic that can do it for you.

This immersion usually comes from gaining access to the pump. As mentioned above, the fuel tank can conceal the pump, so it usually has to come off and that requires draining the gas into a safe container.

If you’re working with a vacuum pump, the diagnosing is fairly simple. You will need some extra vacuum hose and a way to create a bit of vacuum or pressure. Some people will just use their mouth to simulate engine vacuum, but another method for those faint of heart would be using a syringe or a handheld vacuum pump.

Access the vacuum line on the pump and attach the extra line. This is where you will simulate the engine vacuum. Determine which fuel line is the output line. Apply some alternating vacuum/pressure on the vacuum line and feel for responding pressure on the output line.

Some people might watch for the flow of fuel from the output line. If you have drained all the fuel, you might put a piece of paper near the output line to visually simulate a flow.

If fuel comes out of the vacuum port, then the diaphragm is broken. If there is no flow but you can tell that the vacuum is working, there may be some debris blocking the flow. Another culprit could be a vacuum leak, which would keep the vacuum valve closed.

If you are looking at a failing electrical fuel pump, you will want to check the wiring. Make sure the fuse isn’t blown and that it’s the correct amperage rating. You can also check your relay by listening to it click on and off. Look for any broken wiring or other issues that need repairing.

One of the best ways to nail down a bad fuel pump is to flow test it. This is a test designed to measure the output of the pump over a specific time. Not all pumps will flow at the same rate. Performing this test requires having a known good value to compare against. 

To perform the test, have a bit of fuel in a container that can be fed into the pump. Have a line that comes out of the pump into a container with measurements marked on the side (think measuring cup or beaker). You are also going to need to rig up the pump so that you can turn it on and off at will. This can be done by jumping wires to get it to turn on with the key. 

The specifications will be something like the volume of fuel per 5 seconds. Again, refer to manufacturer specifications for details. There is a small margin between a fully-functioning pump and a weak pump, so even if your measurements are close, it might not be enough. 

Once you have verified the pump is bad, it’s time to replace it. Make sure you purchase OEM parts for the best performance. Assuming you have accessed the pump to test it, simply disconnect any hoses and wiring and install the new pump. Also, replace the fuel filter. From there, you can retest the new pump or put everything back together. Add some fuel to the tank and start it up to verify the operation of the engine.

How To Maintain Your Fuel Pump

One simple way to maintain your fuel pump is to be aware of how long it has been since replacement. Age is the biggest reason a pump will fail. They don’t need to be replaced often, but it is a component that will wear out eventually. Record the mileage when your pump gets replaced to be able to estimate if and when it needs to be replaced again.

Another reason why pumps fail prematurely is due to overheating. Fuel pumps are designed to be cooled by the fuel flowing and absorbing excess heat. When the motorcycle runs out of fuel, the risk of overheating the pump is higher because fuel cools the pump as it passes by. Make sure to keep an adequate level of fuel in the tank to prevent this from happening. 

As with anything, a regular visual inspection of the bike is a good way to maintain it. Specifically to the fuel pump, you will want to look for fuel leaks, which should be apparent. Some less-obvious conditions to look for are cracking fuel lines or cracked vacuum lines. Some fuel filters are visible and will appear dark or dirty when needing replaced.

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