Kind of like your drawing only I (for a client) used a no bypass regulator to the fuel log. on the far end of fuel log to keep the fuel colder I ran a 3/8th steel return line to the tank. I inserted various holley fuel jet sizes working as return restrictors until i could maintain about 5 psi.

Why 5 psi...... or even 4 psi? Years ago I had a needle and seat stick open. I got to looking at it and it was hammered from opening and closing. I got to looking at higher quality needle and seats like titanium and SS used for alcohol. They are larger diameter for higher flow rates. Well go down on psi and up on diameter and you have a less aeration to the fuel in the bowl. I did not want to hammer my needle and seats with constant opening and closing.

Years later I was at a race carb shop and the NASCAR guys had been doing the same thing only they had improved the output side of the seat and sold them as an anti aeration needle and seat.

My hot rod uses a stock return type mech pump and I have less than 4 psi and my 830 double pumper has never hiccuped even on WOT on a 3000 foot main straght. it is like on the floor for two 1/4 mile runs

 

Exactly where in your picture is the fuel pressure sensor?

 

I accidentally left that out didn't I?

It is located on the fitting entering the primary float bowl on the fuel log.

 

OK so…if you just block off the return line back to the tank at the regulator will the pressure stay up??? That should at least rule out the pump not performing well. This should be easy to find..just keep removing or blocking things until you find where the change happens….

 

OH one small idea as well…I have seen excess thread tape cause fuel to be restricted or blocked especially in your type of setup…possibly even tearing off inside the line causing a blockage.

 

Good info James. No Teflon tape was used anywhere.

 

I have been bogged down in a legal matter. I have yet to get to work on the car any. I may be able to get to it in the am tomorrow.

I think one of your suggestions will locate the problem. I'm excited to pin this down.

 

Ok, I'm thinking out loud here.

You did not say what you had the pressure regulated to. 5,6,7psi? Anyway you have that pressure going to the carb and pressing against the inlet needle and seat. What ever that pressure you have set is the maximum you can have, but not necessarily the lowest. The specs say that the pump free flows 150gph. That is under no pressure.

So you set the regulator to 5,6,7 psi at idle. The needles are basically closed against the seats. When you are under hard acceleration the floats drop quickly, needles open and fuel flows into the carb to keep up. The pressure is going to drop. That does not mean there is no flow however.

Think about your garden hose. You have 35-50 psi maybe available. When you open the nozzle the water comes out fast at a high pressure. Now take the nozzle off the end and turn the water back on. You still have volume, actually more, but you have no pressure.

I think if you had a 30-40 psi pump you would not be seeing the pressure drop to zero as you would have a lot of "pressure" overhead.

AS an experiment, put some rags under both fuel bowls and remove one of the bottom screws from both bowls so that they can drain. Put them back in. Then have someone turn on the ignition/pump so you can watch the pressure gauge and see what it does as it fills the bowls. I think it may be below your set pressure but as I said, I'm just thinking out loud here.

 

With NO restriction in the return line I'm only getting 2 psi and the regulator will not set. Even if the adjustment screw is all the way in. When I restrict the return with an orifice my pressure goes up at the carburetor BUT will drop when I accelerate hard. The regulator still will not adjust with an orifice in the return line.

This leads me to believe (after the discussion here) that I have...

1. A restriction in my supply line caused by a collapsed line or dirty filter.

OR

2. The pump is simply not putting out adequate pressure due to low voltage or an inadequate supply of fuel.

I think we are getting somewhere now.

 

I finally got to work on the car this morning. Here is what I found out...

The fuel flow through the front 40 micron filter was quite adequate (prior to regulator).

The fuel flow to the carburetor was very low (after the regulator).

Blocking the return line did not affect the situation.

Very low fuel flow through (12-841) regulator to carburetor even when the adjustment screw was bottomed out.

I replaced the regulator with a non-bypass regulator (12-803). I blocked one of the two outlet ports. So basically I just have an inlet and outlet connected to the regulator.

This setup proved to work. Providing 7 psi at idle to the Autometer sensor per the Autometer gauge with pressure adjustability.

So I have a deadhead system setup at the moment. I'm about to go for a test drive shortly to see if there is any pressure drop. I will report back soon.

 

I would plug off the regulator return line and use the. 020 connected to the end of the fuel log

 

I think that is great advice gkull. I need to get the parts together to do that.

I took a test drive this morning after setting the new regulator to 7 psi at idle. The lowest it ever dropped during the drive was 5 psi.

 

The problem with a free flowing 3/8ths return line is the amount of fuel being shot back in tank creating vapor and just evaporating your fuel away. Restricted return keeps the fuel pressure up in the log. keeps the fuel cool, and limits the return flow to a trickle.

 

What he said plus fuel heating.

Can use a fuel pump speed controller to offset both but starting to get impractical for a basic system.

 

Glad you figured something out. Go look at the article I linked earlier. Lars solution solves the vapor lock issue that some have with a dead head system. It would be very easy to do with a Holley style carb and fuel log and will work with the pump, regulator, and return line you have. I was wondering though, why you use a regulator with a 7 psi pump and set the psi to seven. Does the pump produce greater than 7 psi?

 

I've ordered the parts to fix a bypass/restriction after the fuel log . My pump maxes out at 14 psi. Thanks for your help during the process.

 

gkull,
I purchased some .020 drill bits. Man those things are small. I think Lars was using larger orifices in his setup. Just curious why you recommended such a small orifice instead of something like .040 or .060? I realize that it is basically a pressure reliever more so than a volume return orifice but it's so small compared to the others.

 

You really only need to keep some flow through the regulator to keep it from dead heading. Excess flow only makes more vapor back in the tank.

 

What is your recommendation on orifice size. cardo0?

 

CARDO0 answered it. You want tiny flow just to keep your fuel log cooler. You use the regulator to keep 5 psi or so. Use bigger diameter Alki needle and seat to maintain bowl level. .... not higher press that aerates the fuel in the bowls