Fuelpumps

Mechanical fuel pumps are generally mounted on the side of the engine, and work by drawing fuel out from the bottom of the tank using a suction process. A diaphragm creates the suction, and a valve stops the fuel from flowing back into the tank once the pump has filled up.
A mechanical-pump drive hardly ever fails — though signs of wear may appear on the cam and operating lever , linkages and pins. Diaphragms can leak. A small leak may affect the engine noticeably only at high speeds, but may also let petrol drip through into the camshaft area and from there to the sump . If this occurs, there is a risk of an explosion. Valves have a disc and a seating held together by a small cage. Pres- sure in one direction pushes the valve on to its seating, closing it; pressure the other way lifts the disc and opens the valve. Valves last many years, but dirt can get in and stop them seating properly. 

Checking the pump operation
- Seal the outlet pipe(s) with a finger, and push the lever up; it should move freely and the diaphragm should stay still if the inlet valve is seating.
- Seal the inlet pipe and push the lever again; you should feel resistance, but the diaphragm should not move if the outlet valve is seating.
- Seal off all pipes and push the lever again; you should feel resistance if the diaphragm is sound and properly sealed.
- Seal the vent hole in the underside and push the lever, you should feel resistance if the oil seal between the linkage and diaphragm is intact.

Do not test the pump by blowing compressed air into it: you may burst the diaphragm.

Even if you find the fault, you may not be able to mend it. If the pump is sealed, it will have to be replaced.