[TX3100Guy]

Originally Posted by TX3100Guy

I'm not certain this accomplishes anything.

I stand corrected. I've been reading the materials pointed out in post #14

See this photo from that site along with this information.




The above diagram shows the modifications required to a standard mechanical fuel pump to allow it to be used in conjunction with the McCulloch supercharger. Part numbers referred to in the diagram are McCulloch part numbers and modern equivalents will have to be found in order for the modification to be conducted. Mechanical fuel pump modification is required because it is necessary to increase the fuel pressure when the engine is being supercharged in order to force fuel into the carburetor when the carburetor is pressurized. To accomplish this, air pressure from the supercharger is directed beneath the fuel pump diaphragm to help the spring pump fuel at a high enough pressure.

To modify the fuel pump the fuel pump needs to be disassembled sufficiently enough so that the fuel diaphragm can be removed. A 5/16” hole should then be drilled into the side of the pump, where the diaphragm chamber vent holes are located, to allow the supercharger pressure fitting (which is basically a pipe nipple fitting) to be inserted. The location of the hole should be such that it provides direct pressure feed into the air chamber behind the diaphragm. The pressure fitting can be fixed either via tapping the drilled hole (assuming there is sufficient depth of metal to accept a thread), or via a nut inside the diaphragm air chamber which screws onto the pressure fitting. In both cases a rubber sealing gaskets should be used to ensure an air tight connection of the pressure fitting to the fuel pump. Any vent holes remaining after the nipple has been installed will need to be sealed, and this can be accomplished either by using an epoxy resin, or via use of lead shot.

An additional seal (102305) is indicated at the base of the diaphragm air chamber. This was originally a disc shaped composite of metal and rubber and is designed to prevent pressurized air leakage into the shaft where the diaphragm stem connects to the pump drive arm. This can be reproduced using a rubber disc with a central hole which has a smaller diameter than the diaphragm shaft, thus allowing a tight seal against the shaft when the diaphragm shaft is pushed through it. A metal disc which has a larger central hole should then be placed over the rubber disc. This metal disc is held against the rubber disc by the diaphragm spring, and prevents the rubber disc from lifting when the pump drive lever pushes the diaphragm shaft upwards. The central hole should be sufficiently large enough to allow the rubber disc to flex with the diaphragm shaft movement, and small enough to provide a contact point for the diaphragm spring.

Dependant upon the pumps capability it is sometimes necessary to add an additional spring within the existing diaphragm spring, to increase the output fuel pressure from the pump. It is also possible that high pressure replacement pumps are available, or can be built up due to the interchangeability of fuel pump components. The fuel pressure check section described later provides an example of the required fuel pressures for specific boost pressures. Determining the required spring rate in order for the mechanical fuel pump to meet these required fuel pressures is pretty much a trial and error exercise unfortunately.