How To Fix A Mechanical Fuel Pump On A Classic Car

How To Fix A Mechanical Fuel Pump On A Classic Car

This guide captures ‘bygone’ advice on how to Fix A Mechanical Fuel Pump On A Classic Car. 

To see all my ‘bygone’ advice guides please visit Classic Car Maintenance DIY Guides.

Remember folks these are ‘bygone guides’ … useful but the safety and personal protective equipment measures are reflective of bygone awareness. Stay safe.


Mechanical fuel pumps can fail in several ways. They can become loose on the engine block, they can become blocked with sediment, or the diaphragm assembly can wear out.

Mechanical pumps are operated by an egg-shaped lobe on the engine camshaft (as shown below). As the lobe rotates, it raises or lowers a rocker arm on the pump which causes a diaphragm to suck in or push out fuel.

How Does A Mechanical Fuel Pump Work On A Classic Car
The pump rocker arm engages on an egg-shaped lobe on the camshaft and must be correctly located on reassembly.

Retightening the pump with a replacement gasket between it and the engine will usually remedy the fault.

A blocked pump is also reasonably easy to clear. Blockages tend to arise because no petrol is ever per­fectly sediment-free, and often more dirt enters the fuel system than the filters can cope with.

Cleaning the pump filter should be done as a matter of routine.

While cleaning the filter, have a good look at the gasket between the filter bowl and the pump body. If this is faulty, air will be able to enter the pump and give rise to airlocks in the system, so it should be replaced.

How To Check A Mechanical Fuel Pump Operation

The pump’s operation should also be checked. This can be done with the pump on the engine if there is a hand-priming lever (See Below) on the pump.

How To Manually Prime A Mechanical Fuel Pump On A Classic Car
This AC mechanical pump has a hand priming lever. If you operate it with the inlet closed off you should feel suction.

To remove the pump disconnect and plug the fuel pipes and remove the retaining bolts. Retain any pack­ing washers between the pump and the cylinder block as these affect the pressure output. The gasket between the pump and engine should also be replaced on reassembly.

Whether the pump is on or off the engine, the pro­cedure for checking it is the same. First place a wetted finger over the inlet so that it is completely blocked off.

Next, operate the pump by either moving the priming lever upward or the rocker arm inward.

A suction force should be felt on the finger. Repeat the operation, but this time close off the outlet. This time pressure should be felt.

Testing Fuel Pump Suction ON A Classic Car Mechanical Fuel Pump
Operate the lever again but this time close off the outlet. If the pump is working you should feel pressure

If there is no lever the pump will have to be removed and the rocker arm used (See Below).

How To Prime Mechanical Fuel Pump With No Prime Lever
lf you have a pump which does not have a hand priming lever to operate the pump you will have to move the rocker arm

Testing Fuel Pump Suction ON A Classic Car Mechanical Fuel Pump
The arm may be hard to move so if you do not feel suction or pressure it may be that you are not moving it far enough.

If very weak suction or pressure is felt, or none at all, the pump is faulty.

If the cause is a blockage, clear it by blowing air through the inlet. If retesting shows that this cure has worked carefully replace or overhaul the pump by the method given below.

How To Overhaul A Mechanical Fuel Pump

Before proceeding with a pump overhaul it is worth checking with a dealer to find out exactly what replace­ment parts can be bought for your particular pump. For while kits are available for many types of pump some cannot be repaired and so must be discarded.

The following may help.

Latest Mechanical FUEL PUMP Listings On eBay

If it is possible to overhaul your pump first remove it from the engine.

Next undo the screws holding the two body parts together. Carefully lift off the top part.

Delicate use of a thin knife may be necessary to prise the two apart. The diaphragm assembly will now be exposed.

How To Overhaul A Mechanical Fuel Pump On A Classic Car
A faulty pump can be dismantled by undoing the retaining screws which secure the two halves of the pump together.

How To Overhaul A Mechanical Fuel Pump On A Classic Car
With the screws removed the top part of the pump can be removed. This reveals the diaphragm seated in the lower half.

How To Replace The Diaphragm On A Mechanical Fuel Pump

To remove the diaphragm turn it by its metal centre a quarter turn and it will lift clear.

Examine all the parts now visible.

If any of them, such as the diaphragm or its spring appear worn or damaged they should be replaced.

How To Overhaul A Mechanical Fuel Pump On A Classic Car
To remove the diaphragm in order to examine it grasp it by its centre press it down and twist it 90°
How To Overhaul A Mechanical Fuel Pump On A Classic Car
Remove the diaphragm and spring. Check them for wear especially the diaphragm which often splits or perishes.

Before fitting a new diaphragm should be soaked in fuel for about 15 minutes.

When reassembling the pump the diaphragm spring should be put back into the bottom part of the pump assembly with its larger diameter, if any, uppermost.

Rest the diaphragm on the spring with the flattened end of link the pull-rod in line with the slot in the link connecting (See Below).

How To Change A Mechanical Fuel Pump Diaphragm On A Classic Car
On reassembly be sure to locate the flat end of the diaphragm assembly in the slot in the lower part of the pump.

Move the rocker arm so that the moves to the top of its travel and then push down the centre of the diaphragm until it enters the link slot.

Give it a quarter turn to lock it.

Move the rocker arm until the diaphragm is level with the pump body and then replace the top part of the pump.

Finally, push the rocker arm inwards to its full extent and tighten the screws holding the two parts together.

Once the pump has been reassembled replace it on the engine. Take great care that the cam eccentric engages the pump rocker arm- that is the flat surface of the arm must rest against the cam ( As shown in the first picture in this post). Incorrect replacement could lead to the breakage of the pump arm-and possibly cause serious engine damage.

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