Modifiying fuel delivery

[Roger/Sacramento]

I don't have a regulator on my system. I don't think I have a problem with too much pressure but, am thinking about doing something to make sure I don't. Right now I have a stock mechanical and a four lb. electrical backup. To install a regulator between the mechanical and the rear carb would be a real mess ( look at the picture ). How about if I disable the mechanical and leave it in place and install a regulator set at 2.5 lbs. in front of the electrical which is back by the fuel tank ? How should i disable the mechanical and not be concerned about fuel going into the block ?

[Vic Piano]

Why not just put a block-off plate on the intake manifold and use a fuel log for delivery to the carbs with the electric fuel pump as the primary?

[ford38v8]

A regulator works well at the exit of the electric pump. To keep the mechanical pump inline for aesthetics is very common, requiring nothing special at all. I suppose some remove the pushrod, but not neccesary. The only way you'd get fuel into the block would be if the diaphram ruptured, but with a modern rebuild kit that's not too likely.

The big disadvantage to keeping the mechanical pump inline is the heat buildup within the mechanical pump, being at the top rear, it absorbs heat from the engine below as well as the air above. Todays fuel having a lower boiling point, vapor lock is much more prevalent than in past years.

The electric pump online will overcome the vapor lock issue, but the worry then becomes boiling in the carb itself. Fuel spillage inside and outside the carb at shutdown, the resulting mess, and empty float chamber when returning to restart is a pain.

A 1/8" return line having a .070" restrictor installed after the mechanical line wouldn't be too noticeable, and would help keep the fuel cool enough to remain a liquid. At engine shutdown, this line would also insatanly relieve the fuel pressure before the carb, helping to prevent the overfilling of the bowl with the resulting spillage.

[trainguy]

would the return line go back to the tank?

[ford38v8]

Originally Posted by trainguy

would the return line go back to the tank?

Yup. Gotta have the restrictor up front, though.

[G.M.]

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Originally Posted by trainguy

would the return line go back to the tank?

Return the fuel into the filler neck. If a .125 ID return is slightly large (I'm thinking .100) You could put a small restrictor in the line. The air line on the early fuel gauge line may be just about right. Stromberg carbs have a .098 restrictor hole in the inlet in the needle valve. As mentioned before I found on 100 degree days about 55 MPH, no vapor lock and it starts below 55. The reason is more fuel is flowing through the pump cooling it. The return would allow more fuel to flow through the pump even at idle and I "THINK" will prevent vapor lock. G.M.

[gmc1941]

Would the 1/8 return line be sufficient enough or would you still need the restricted ?

[Roger/Sacramento]

The point I made is being missed. I don't want to disrupt the stainless line-banjo fittings from the mechanical pump to the rear carb. I really want to know how to deactivate the mechanical pump without causing fuel to drop into the engine.

[G.M.]

Originally Posted by gmc1941

Would the 1/8 return line be sufficient enough or would you still need the restricted ?

There is no sure way to determine what size return line would be required until tried. A 1/8" or .125 hole may work. I believe the hole will be from .100 to .125. If the hole gets to big the engine will starve for fuel at high speeds. I have my 39 set up to add a return line. I will try it next month when I get back to Florida it will still be over 90 there. I'll get it good and warm and get on I-75 and see how it runs then get into traffic and see how the VL acts. G.M.

[Bluebell]

Roger, to answer your question, you can strip the mech pump, and throw the diaphragm away. You could replace it with a metal plate, and gasket, but it would be better to replace it with a sheet of material that currently is used to make diaphragms Or use a modern diaphragm and cut the push rod off. The metal plate would add to the vapor lock issue. A suitable sheet of viton (read elsewhere on this forum) would be good. You could plug the base of the pump to further prevent fuel getting down in there. Leave out the valves, spring levers etc.

[Pete]

You have 3 carbs to add performance to your engine so at some point in time you are going to try running at full throttle if only for a short time.
With the plumbing you have, you will be short on fuel to the middle and front carbs.
That is the worst possible way to plumb 3 carbs.
HOWEVER, it will get by on the street or for shows.
For the street, you can run through the stock pump ok.
If you think you need a regulator, put it back by the electric pump but put a gauge up front to check the pressure.

[Roger/Sacramento]

Bluebell and Pete....thanks for the info. I have better information now to decide the fix.

[Mike51Merc]

Nice setup you got there.
This conversation is built upon an unproven assumption. First you should measure the pressure the mechanical pump is putting out. If it's a bit too high, install an extra gasket or two under the pump stand to lower it.

Secondly, most mechanical pumps will allow the electric pump to flow through and you can mount the regulator and a small pressure gauge on the firewall.

[Roger/Sacramento]

Mikes51merc.....my electric fuel pump is rated at 4 lbs. so I need to reduce that, ergo a regulator in front of it. Also I don't want to modify the fuel line from the mechanical pump to the rear carb.