How the motorcycle fuel pump works

For an internal combustion engine to function, we need three elements: air, fuel, and spark. In a carburetor engine, fuel is drawn in by the vacuum generated inside the engine. In modern EFI engines, fuel is injected under pressure in the correct amount and at the precise moment into the combustion chamber.

Under pressure

The fuel pump is used on motorcycles with electronic injection (also on some carburetors with remote fuel tanks) to supply the injectors with a constant supply and at the appropriate pressure.

The pump components are usually located submerged in the fuel tank (this cools them and muffles the noise of their operation). The fuel passes through the pre-filter and then, once pressurized, into the filter.

This filtering phase is essential to eliminate solid residues that could damage the injectors.

Once filtered, the fuel passes to the pump, where it is pressurized. This component normally operates electrically, with 12 volts supplied by the injection system control relay. There are various types of pumps (rotor, turbine, lobe, roller, etc.), but their function is to transfer gasoline from the tank to the injectors.

They usually work continuously, providing a constant flow that increases fuel pressure.

The regulator

The pressure regulator is a mechanical system, composed of a spring and a valve, that eliminates the excess pressure generated by the pump, returning some of the gasoline back to the fuel tank.

This ensures a constant fuel supply, both in terms of flow and pressure. This is very important because we always obtain the same amount of gasoline each time the injectors open, regardless of engine RPM (the injector will then supply more or less fuel depending on how long it remains open).

A fuel level gauge is also usually integrated into the pump system, which usually operates using a variable resistance float or ultrasound.

The high pressure connector

Normally, pressurized gasoline is delivered to the injectors by a reinforced hose,

connected directly to the injectors at one end and to the regulator outlet via a quick connect located on the outside of the fuel tank.

Breakdowns and diagnosis

There are two main types of fuel system failures: electrical and mechanical.

The most common electrical faults are a lack of voltage in the pump circuit or overheating of internal components, which can cause the pump to melt. To prevent overheating, it's important to avoid driving frequently with the fuel level too low (remember that gasoline cools and lubricates the pump components).

On a mechanical level, the pump's internal components often fail due to wear. Regulator failure is also very common, causing a loss of pressure in the system. Similarly, the internal hoses can become perforated due to friction with the tank walls, creating pores and pressure loss.

The most commonly used instrument for checking fuel system operation is the pressure gauge, which is connected to the pump outlet, before the injectors.

The pressure must be constant at all rpm ranges and must be within the pressure specifications specified by the manufacturer in the workshop manual for the specific year and model.

Frank Burguera

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