47-55.1 Does this look familiar to anyone?

[leegreen]

The pressure in the fuel system has to be higher than the pressure surrounding the carb. Unless there is higher pressure right next to the supercharger outlet the most likely flow in that line is from fuel pump towards supercharger. From either side of the pump you'd get fuel at that line. 1/8" of fuel squirting into the the intake would be a noticeable problem.

If the hole drilled in the pump is near the mounting boss of the pump it might in affect be venting boost pressure into the engine block. That does not make sense.

Both options are weird. This engine was built back in the days when little was known about turbo or supercharging in the hot rod world. The line may be a mistake

[TX3100Guy]

Here is a photo of me holding the mechanical fuel pump up with the hose connecting the nipple inserted into it, leading to the supercharger boosted air output. You can see the small hole in the wall of the supercharger that feeds this 1/4" rubber hose.

[leegreen]

if you blow air through hose where does it come out of the pump?

[TX3100Guy]

Originally Posted by leegreen

if you blow air through hose where does it come out of the pump?

Curiously, it doesn't seem to do anything. It seems to pressurize the mechanical fuel pump housing and it slowly leeks from the seams of the part. When I pull away my air hose from the pump, there is a rapid release of the air pressure from the drilled and tapped fitting. I'm with you, I'm not certain this accomplishes anything.

[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.

[leegreen]

I'd never realized that mechanical pumps used the spring to power the pump cycle and the cam arm was the return stroke. But is does make sense, the spring gives some pressure regulation, if it was the other way there would need to be a pressure relief.

So now the boost pressure bleed to pump makes sense, the spring needs the extra pressure to get the fuel pressure high enough for the carb when it is under boost. Now the electric pump makes sense too, it turns on from the manifold switch to put enough pressure on the intake side of the mechanical diaphragm to overcome he boost pressure and fill the diaphragm chamber. Without the electric pump the diaphragm would flex away from the boost pressure and the pump volume would fall off.

If you are replumbing anyway, bypassing the whole works in favor of a modern electric pump, filter, non blocking regulator and return line to tank is probably a more reliable approach. less historically interesting but your truck, your call.

[TX3100Guy]

Originally Posted by leegreen

I'd never realized that mechanical pumps used the spring to power the pump cycle and the cam arm was the return stroke. But is does make sense, the spring gives some pressure regulation, if it was the other way there would need to be a pressure relief.

So now the boost pressure bleed to pump makes sense, the spring needs the extra pressure to get the fuel pressure high enough for the carb when it is under boost. Now the electric pump makes sense too, it turns on from the manifold switch to put enough pressure on the intake side of the mechanical diaphragm to overcome he boost pressure and fill the diaphragm chamber. Without the electric pump the diaphragm would flex away from the boost pressure and the pump volume would fall off.

If you are replumbing anyway, bypassing the whole works in favor of a modern electric pump, filter, non blocking regulator and return line to tank is probably a more reliable approach. less historically interesting but your truck, your call.

Thanks again for the input. I'm not terribly interested in keeping thing historically correct since the truck is obviously modified with the supercharger, but I am interested in updating the entire fuel system to ensure it is ethanol ready and capable of running efficiently.

I've already purchased a new fuel tank that is going to be mounted in an old steamer truck in the back of the bed. I have the new mechanical fuel pump on order from Amazon per the link provided above. Now I need to acquire the modern electric fuel pump and a non-blocking regulator.

The only unanswered questions I have are 1) what psi would be required of both the electric fuel pump and the regulator. Many of the pumps and regulators are for 100 psi fuel injected modern engine. I'm unable to determine anything from the regulator and the fuel pump since neither has a single marking on it for brand or model. Here is a picture of the old fuel pump, I've already posted photos of the regulator.

Also my original plan was to use braided stainless flexible hose from the tank to the electric fuel pump, filter, and mechanical fuel pump. I read an article this morning suggesting that was a bad idea and that I should plan on using hard lines. A friend who used to own a shop restoring classic cars calls BS on that and said they used braided stainless all the time for both fuel feed line and return lines.

[dsraven]

i would say your fuel line pressure at the carb inlet should be around 4 to 8 psi, otherwise the inlet needle and seat in the carb will be overcome and you will/can flood the engine. a lot of gallons per hour is also probably not required. you should be able to find one for around $50-75. you might think about using a fuel additive as these old engines were built when we had lead in the fuel which acted as a lubricant on the valve seats in the cyl heads and also as a detonation inhibitor. lots of guys used to retard the timing a bit to stop detonation when using unleaded gas. of course a hard line set up for the fuel lines would be optimal but there are lots of vehicles running around with the braided stainless lines for short line runs where there needs to be some flex built in. braided stainless lines are usually a PTFE tube wrapped inside the stainless braid. PTFE,Polytetrafluoroethylene, has less surface friction than rubber, which means that flow can be improved by using PTFE hoses. While rubber is prone to break down at extreme temperatures, PTFE is highly temperature resistant, making it ideal for all kinds of automotive uses. most fuel or hydraulic hose shops will tell you the steel/hard lines will flow better because they have less friction loss than a rubber or PTFE line and the hard lines are less prone to deteriorate over time. the stainless braid helps keep the hose liner from expanding while also providing some mechanical protection.

[leegreen]

remember you need a regulator that can compensate for boost, you need to overcome the float pressure plus current boost psi.

I'd go with hard lines myself leaving just the inches to the mech pump as flexible. Hard lines are easier to install, you only need to support it every 18" or so and once done it will last the life of the build.
Flex lines just look wrong unless they are supported more often. And so much of what passes as SS Flex these days is just cheap hose with a thin braid slid over it.

[TX3100Guy]

Originally Posted by leegreen

remember you need a regulator that can compensate for boost, you need to overcome the float pressure plus current boost psi.

I'd go with hard lines myself leaving just the inches to the mech pump as flexible. Hard lines are easier to install, you only need to support it every 18" or so and once done it will last the life of the build.
Flex lines just look wrong unless they are supported more often. And so much of what passes as SS Flex these days is just cheap hose with a thin braid slid over it.

I'll look into hard lines. I have to run new brake lines too and they will be hard lines, but the steel ones are challenging to bend.

[TX3100Guy]

Originally Posted by dsraven

i would say your fuel line pressure at the carb inlet should be around 4 to 8 psi, otherwise the inlet needle and seat in the carb will be overcome and you will/can flood the engine. a lot of gallons per hour is also probably not required. you should be able to find one for around $50-75. you might think about using a fuel additive as these old engines were built when we had lead in the fuel which acted as a lubricant on the valve seats in the cyl heads and also as a detonation inhibitor. lots of guys used to retard the timing a bit to stop detonation when using unleaded gas. of course a hard line set up for the fuel lines would be optimal but there are lots of vehicles running around with the braided stainless lines for short line runs where there needs to be some flex built in. braided stainless lines are usually a PTFE tube wrapped inside the stainless braid. PTFE,Polytetrafluoroethylene, has less surface friction than rubber, which means that flow can be improved by using PTFE hoses. While rubber is prone to break down at extreme temperatures, PTFE is highly temperature resistant, making it ideal for all kinds of automotive uses. most fuel or hydraulic hose shops will tell you the steel/hard lines will flow better because they have less friction loss than a rubber or PTFE line and the hard lines are less prone to deteriorate over time. the stainless braid helps keep the hose liner from expanding while also providing some mechanical protection.

Thanks for the information.

[TX3100Guy]

Originally Posted by leegreen

remember you need a regulator that can compensate for boost, you need to overcome the float pressure plus current boost psi.

I think this unit answers all of my needs. Any concerns?

https://www.speedwaymotors.com/Holle...alQuery=12-881

[fauXGT]

Originally Posted by TX3100Guy

I think this unit answers all of my needs. Any concerns?

https://www.speedwaymotors.com/Holle...alQuery=12-881

Attachment 2190868

Read the spec sheet.
It looks like it has the vacuum referenced adjustment for boost.
Not the same part, but same principle what I put into a TBI truck with aftermarket supercharger. The more you put your foot into it, the higher the fuel pressure and the more fuel offered. Only difference is I have a modern blow off valve.

[leegreen]

Looks like you can use it in boosted application.

The diagrams in the specs cover a few different models. Blocking and nonblocking, not sure which this specific model is

You don't have a return line now, but I think if I was re-plumbing anyway I'd want one. Electric pumps like having flow for cooling. The way your truck is wired it appears the electric pump only runs under boost so it may not matter much, but decide if you want want a return line and non blocking regulator or no return line and a blocking regulator