How does a motorcycle fuel pump work?

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The fuel pump transfers fuel from the tank to either the carburetor, or the fuel injector. When the fuel tank is below that of the carburetor or the force gravity is not sufficient to transfer fuel, a pump is needed. This includes fuel that is stored under the seat, or close to the carburetor.

What is the purpose of a fuel pump? The fuel pump is a diaphragm that moves and creates pressure and vacuum. The flow is controlled by check valves, which only allow one-way flow. Pressure and volume flow are controlled by the geometry of your diaphragm as well as the speed of your pulse.

Fuel systems are designed to produce specific flow and pressure, which requires specific fuel pumps. The pump can be hidden under the tank’s frame. Some pumps are located within the fuel tank’s bottom. This location can vary so make sure to check the specifications of your bike.

What is the Work of a Fuel Pump?

Two types of fuel pumps are available for motorcycle engines: electrical and vacuum. These are not interchangeable. However, there are many ways to make a vacuum pump an electrical pump. Both serve the purpose of the engine’s specific design. Modern motorcycles require an electric fuel pump to provide the fuel injection pressure.

Vacuum pumps work in a different way than you might think. The engine vacuum is created by pulses of a diaphragm to control a fuel pump. Since the vacuum of the engine has a bit of a pulse, the pump can flow fuel to the carburetor through all RPM’s. The vacuum from the engine is transferred to the fuel pump via hoses and fittings.

An electric pump is simple. Instead of the engine vacuum pulsed by a diaphragm this is done by an electric motor, or a solenoid. Motors spin when an impellor pushes fuel. This effect can be created by a solenoid, which turns an electrical current on/off and pulses the diaphragm upward and downward.

An electric pump will also include a fused relay. Circuits that are shorted to ground can be protected by a fuse. Relay is a switch that uses small amounts of current to control a larger current. This allows the low-voltage ignition switch to control the pump.

Signs that the fuel pump is failing

There are many reasons why fuel pumps fail, and several ways to diagnose them. Here are some signs that your fuel pump is failing:

  • Engine will start, but it won’t crank.
  • It will be difficult to start the engine
  • Poor performance in acceleration (stumbling).
  • Your engine will either surge, or sputter.
  • Poor idling
  • Engine power loss overall
  • Engine will stop
  • The pump makes a whining sound

Three things to look at if your engine refuses to start or has difficulty starting are 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. It is common sense to check that fuel is in the tank. If there is and the engine still isn’t getting fuel, it could be the pump is going bad.

You may notice a weak acceleration or stumbling when the bike starts. This could be due to the pump not working properly. The pump will eventually wear out and become inefficient at keeping up with the demands of the engine. Simply put, it’s not getting enough fuel to the carburetor. Low fuel flow will cause surging, stumbling and sputtering. Sometimes, the engine may stop running completely.

You may hear a whining sound coming from your pump. The pump will wear out faster, creating greater tolerances between moving parts. This can lead to more vibrations and a whining sound.

How to replace or fix a fuel pump

If you start to notice any of these symptoms, it’s time to start doing some diagnosing. Pinpointing a bad fuel tank can be a difficult task, so plan ahead. If that isn’t an option, call around to find a competent mechanic that can do it for you.

This immersion is usually achieved by having access to the pump. As stated above, the fuel tank may conceal the pump. To drain the gas, you will need to remove it.

If you’re working with a vacuum pump, the diagnosing is fairly simple. A vacuum hose will be needed to add some pressure or vacuum. Some people may just use their mouths to simulate engine vacuum. However, for the faint-hearted, a syringe and a handheld vacuum pump are better options.

Attach the extra cable to the vacuum line of the pump. Here you can simulate engine vacuum. Determine which fuel line is the output. Apply some pressure/alternating vacuum to the vacuum line. Feel for the responding pressure on your output line.

Some may be interested to see the flow of fuel from an output line. To visually mimic a flow, place a piece paper next to the fuel line if you have exhausted all of the fuel.

If fuel escapes from the vacuum port then the diaphragm has been damaged. If you cannot see the vacuum working but there is no flow, it could be that there is debris blocking the flow. The vacuum valve could also be closed due to a leak.

You should check the wiring if you see an electrical fuel pump that is failing. Make sure the fuse isn’t blown and that it’s the correct amperage rating. Relay can also be checked by hearing it click on/off. You should inspect the wiring for broken connections or any other problems that may need to be repaired.

Flow testing can be a great way to find a bad fuel pumps. This is a test that measures the output of the pump over a certain time. Different pumps may not flow at the same speed. This test must be performed against a known good value.

You will need fuel to perform the test. You will need a line from the pump that runs into a container marked with the measurements (think measuring cup, beaker). The pump will also need to be rigged up so you can turn it off and on at will. To get the key to turn it on, you can jump wires.

Specifications will look something like fuel volume per 5 seconds. Refer to manufacturer specifications again for more details. A fully functioning pump is different from a weak one. This means that even though your measurements may be close, they might not be sufficient.

Once you have verified the pump is bad, it’s time to replace it. To get the best performance, ensure you only purchase OEM parts Once you have access to the pump to test it out, disconnect all hoses and wiring. Then install the new pump. Replace the fuel filter. You can then retest the pump or put it back together. Start the engine by adding fuel to it.

How to Maintain Your Fuel Pump

It is simple to keep your fuel pump in good condition by knowing how long it has been without a replacement. Pumps that fail due to age are most likely to fail because of their age. They don’t need to be replaced often, but it is a component that will wear out eventually. Keep track of the mileage at which your pump was replaced so you can estimate when it will need to be replaced again.

Overheating can also cause premature failure of pumps. The fuel flowing through the pump cools it and absorbs excess heat. Because fuel cools the pump, it increases the chance of the engine overheating. To prevent this, ensure that there is enough fuel in your tank.

A regular inspection of the bike is a good idea as a way to keep it in top shape. Particularly, inspect the fuel pump to check for fuel leaks. Cracked fuel lines and cracked vacuum lines are two other conditions you should be aware of. Some fuel filters will be visible and can appear darkened or dirty when they are due for replacement.

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