I'm having problems with my carbed 383 GTA that I recently aquired from a friend. It has a 750 Mighty Demon carb being fed by a Mallory 140gph electric external fuel pump and through a Mallory 4309 return regulator. The supply line is 1/2" and the return is 3/8". I'm getting way too high of fuel pressure that I can't bring down with the regulator adjustment. The wierd thing is though that it started responding finally though I'm not sure why. But now it's high again. Here's the whole story:

I took the induction off of the car after I got it to clean things up and took the carb off and apart just to check things out. When I put it all back together and slightly reconfigured the fuel lines (moved the regulator to the front of the manifold and bent up two seperate 3/8" lines from the reg to the carb bowls and just kinda cleaned up the plumbing including adding a pressure gauge right on the reg itself. Well when turning the pump on the pressure showed 12-13psi constant and would not come down at all no matter how much I decreased the set screw on the reg. It would however go higher if I went in on it though I was sure to not go too high as I didn't want to damage anything. This pressure caused the bowls to over fill and for the engine to run really rich. Also, with the engine running the needle on the gauge would fluctuate wildly and oscilate from maybe 6psi up to 13 or 14 psi. After driving the car a few times and trailering it to another location and after a little more clean up work the pressure seemed to settle down. I could suddenly get the pressure all the way down to 3 psi with screw all the way out on the reg and could adjust to 6-7psi and maintain that. The bowls would respond much better and the car didn't run nearly as rich. Everything seemed fine and I was ideling the car and tuning a bit when I accedentally ran out of gas. The bowls ran dry and the engine stalled. I took the carb off and apart just to check out what jets were in it to get a base line and then put everything back together and added fuel to the tank. I filled the bowls and am now again getting the same high pressure problem. I can't get it to come back down even with the screw out of the reg. I drove it a little hoping it would settel and it of course is running rich again.
I might think this is a return line problem, but if so, why would it have been working so well for a while? Maybe a problem with the pump's bypass setting?

Here are some before and after pics of the setup:
Before:


After:

 

Make sure there is absolutely no restriction in the return line anywhere along it.

I had fluctuating issues with mine as well and could never get the pressure down. One day I found that on my rubber return line on the inside had gotten cut where I screwed the brass barb fitting into it, and there was a flap of rubber inside the rubber line - when I cut that rubber chunk out, pressure came down and stayed there - the flap of rubber was causing a restriction.

You mentioned 1/2in feed and 3/8 return - but I see you are still using the stock hardlines from the tank to the car - the stock lines are 3/8 feed and 5/16 return. If you tried shoving a 3/8 hose barb into a smaller rubber line (like I did), then you could have the same cut rubber flap on the inside of the rubber line.

 

Actually the guy who put the car together used the stock supply 3/8" line as the return, removed the stock 5/16" return line entirly and added in 1/2" push-lock hose from the pump to the regulator. I didn't like the idea of rubber hose near the exhaust and driveshaft so I purchased some straight 1/2" coated steel line and bent it up using the stock 3/8" as a template and that's what you see there that I'm using as the supply right next to the stock 3/8" line. The previous owner also had the stock fuel filter still in the line and had just punched it out, so I got some of those metric adapter fittings and flared just a straight section of tubing and put that in place of the filter. I didn't like the idea of going backwards through the stock filter even if it was punched out...a little too rigged for me.

 

I'm using that same regulator I think and I know mine has a spot marked feed, and return. You sure you got your hoses right?

Also, are you saying your fuel filter is on the return line? Thats not right. It needs to be on the feed line. How do you have the lines going in and out of the tank? You said your friend had the stock feed line as the return line. Does that mean its trying to push fuel through the stock pump?

 

Alright, let me start over. I know basically how a fuel system should be plumbed although it might get a bit fuzzy when it comes to where inside the tank lines should be entering and exiting. My back ground is in EFI but I'm usualy starting from a good foundation. My friend may have been a little crude at setting things up but the results showed it to work. That being said, I wanted to clean it up to avoid any possibilities of problems down the road. To my knowlegd this is the first time this problem has happened(the high pressure).
So, I'll just step through how things are arrainged but keep in mind that the only thing I have changed since this problem came up was changing 1/2" push-lock hose to 1/2" steel tubing starting around the rear seat area, removing the stock filter and putting straight tubing in it's place, relocating the regulator from the side of the manifold to the front and using 3/8" tubing instead of 3/8" push-lock to the carb bowls. I guess the main thing that I didn't change was the pump, the filter at the pump, and the inlets/outlets at the tank.
Tank->hole drilled at the rear with bulkhead 10AN fitting->10AN filter w/ replacable element->Mallory 140 pump->8AN push-lock hose->1/2" steel line->Mallory regulator(4 inlet/outlet ports and one return port)->two 3/8" steel lines to carb bowls->3/8" steel line to 6AN push-lock->stock 3/8" steel line->metric adapters and straight 3/8" tubing(where stock filter used to be)->stock 3/8" line->tank
Now, I'm pretty sure there's no stock TPI pump, at least I give him more credit than that. I want to say I remember him pulling out the sending unit and hacking away at it which makes me wonder if there might be a problem there too. I kind of think he just chopped off the 3/8" a little after the tank sending unit flange. And the bulkhead fitting pulls fuel from whereever it pulls from...that's probably why it ran out of fuel on me at 1/8 of a tank.
So any suggestions where to go from here are welcome. Another thing to note is that this car has been sitting as I got it for maybe 2 years with periodic runnings so I'm half wondering if some things have degraded in some way(pump/regulator).
From looking at the regulator manual I think you can connect the supply to any of the 4 top ports as well as the outlet ports but the bottom port must be the return. If you look into it with the fittings off it's pretty much all one big open cavity in there.
I guess one thing to try would be removing the return line at the regulator and pumping straight into a gas can just to see if that's my problem...

I appologize if I'm being a bit rambly...I just want to get this car on the road and have some fun.

 

I checked out your cardomain site (nice car BTW!) and it looks like we're running different regulators.
Is this yours? http://www.malloryperformance.com/Pr...minselection=4

This is the one I'm running:
http://www.malloryperformance.com/Pr...minselection=4

 

I think this is a good idea. If your starting the car just make sure to use a long enough hose out of the regulator so that the gas can is away from the motor. This will eliminate your return line system so it will give you info. I dont see how it would be in the feed line at all since it is a return style regulator. It has to be either the regulator or return line.

You are correct. We do have different ones. And thank you

 

Well I did some testing tonight and the results seem to point to the return line being the issue, though I'm still stumped. Why did it work that one time for a good while and how it doesn't?
I poked around at the regulator some, opened it up, cleaned it up. It seems to be working right. With the adjustment screw out and the pump running it pops wide open and is dumping as much through the return as it can. When I ran my external hose directly off of the regulator return fitting and ran it into a jug I could bring the pressure down to 3-4psi. When moving the external hose to the next fitting down the line where it joins into the stock 3/8" line it moves up a bit to maybe 5-7psi. When I move down the line again to where the stock filter used to be it moves up to maybe 10psi. All this adding line sure does seem to point to the return line being too small. But why the heck would a 3/8" return line with a 1/2" feed line be too small?
I also took off the ring on top of the tank and peaked inside(previous ower had cut an access hole in the rear deck). It looks like all he did was remove the stock pump and left the 3/8" tube ending where it used to hose into the pump. And I watched fuel get pumped through it into the tank. The stock 5/16" TPI return line is still in there as well ending down where the stock pump hanger is though I think he capped it off somewhere around where that plastic vent valve used to be outside the tank. The pick-up is actually a little more elaborate than I was thinking. There's the 10AN bulkhead fitting going through the tank and then on the inside he has 10AN push-lock hose leading down into the tank into the baffled area, so I guess it's picking up pretty well after all.

Do I really need to run another 1/2" line all the way from the tank to the regulator to get the pressures down? Again...why did it work so well that one time with the same setup I have now. Maybe there's some type of debree moving around and is hung up somewhere in the chain? Also, after all of my fooling around tonight, with everything put back together I'm now getting ~10psi instead of 12-13 with the adjustment screw backed all the way out.

 

Not sure why it worked on time and not another, but I do run 1/2" line from the tank, and back to the tank and have no problem getting my pressure down to 6 psi. I also have a -8 bulkhead fitting on top of the tank that my return is hooked into.

 

Hey 430 any progress with this? I just tried my system with 4309 and stock lines for the first time and have the same problem.

 

I've played around with it a little more. I got to thinking that maybe a restrictor in the supply line would knock the flow down enough that the 3/8" return would be able to handle it. I started out putting a 3/16" ID restrictor (thinking I was starting really small and would have to drill it out to get it where I needed) in the supply up near the regulator(in the union I have under the master cylinder) and all it did was make the pump draw 0.2 more amps (from ~2.6A to ~2.8A at ~12.4v) than without it. It didn't change the pressure at all. I got to thinking maybe the restrictor would only work when the engine is pulling more fuel but at idle the bowls are closed most of the time so the pressure basically stabalizes back to what the restriction of the return line is. Problably what it'll end up doing is dropping fuel pressure at WOT and starving teh engine. I'll be taking it back out...
I am leaving it as is for now since at 10psi I can control my bowl level and I've been adjusting the idle mixture so that it idles fairly well. I'd still like to get it down though so I know it's stable and won't give me problems in the future. I think the first thing I'll do is Tee the 3/8" and the 5/16" at the tank and either run a 5/16" all the way up and Tee again to a single 1/2" line coming out of the regulator, or run a 1/2" all the way back and just Tee once at the tank. I'm going to try that just so I don't have to make another tank penetration. The other option would be to get a smaller pump, maybe the Mallory 110 instead of the 140 since I don't think this engine is needing that much fuel. But running larger plumbing is cheaper at this point.
Trumps2000, so are you running the stock 3/8" as the supply and 5/16" as a return? What pump are you using with that 4309? How high is your pressure?

 

Why not just run a brand new 1/2" return line all the way to the tank with a bulkhead fitting? Yeah you have to drop the tank to put the fitting in, but you know it will work. Plus its the right way and with a fuel system, I wouldnt cut corners.

 

Where on top of the tank did you put the bulkhead fitting?
I was looking through your pics on your cardomain site some more. What kind of 1/2" lines did you use to run the length of your car? Aluminum?

 

I'm running Summit's version of a Holley Blue pump. I think its overkill, but I thought I'd be safe. I might have to jump down to the Red too. What about the spring and plunger in the pump, would playing with that do anything? And yea I'm running stock sized lines. Its seems to hold ~9 PSI with regulator backed out and the motor not running.

On a side not, how loud is your pump. Mine's loud enough to be annoying.

 

Here is where my return line is. You can see it if you look right above the adjuster for the panhard rod. I want to move it though. Right now its not actually on top of the tank. Its towards the top on the back side. I sometimes get a little bit of seepage out of it if I fill the tank. I may wait becuase I really want to make a completely new tank.



And trumps, I have a Summit pump also, dont remember if its 140 gph or a little more but it will go up to 18 psi. Its also very loud.

 

Hey 430 I think I have an idea. I'm going to find a power resistor to drop the voltage on the pump, so its not pushing so hard. I'll let you know how that works.

 

Please do let me know how it works out. I had actually given this some thought because I got to thinking that maybe when I saw the pressure straighten out it was when I had all the accessories off while cleaning them (IE: no alternator) and that the pump was running on a lower voltage. But I don't totally remember if that was the case. I kind of think not. A PWM circuit would be a more efficient way of controlling the fuel pump speed but a voltage drop accross a resistor should work too, it'll just create heat at the resistor.

 

Yea, I was considering buying a fuel pump controller, but when I saw the price tag I thought about designing my own, or at least finding a cheap adjustable power supply. My problem is I'm under the gun to get it inspected.

Any way I tried it with a 2ohm resistor and it worked! I was able to drop it below 3PSI with the regulator fully open then control 5.5PSI flat. Keep in mind my motor wasn't running. Also I should tell you when I tried to pump the pressure up beyond ~7PSI the system freaked out by oscillating. It might have been the wirewound resistor's inductance causing the oscillation, but I won't know until the one I ordered shows up. I think I'll be fine because it won't see more load then the motor not running. The only question I have is what will it do at 14.5V when the motor is running.

 

Wondering how this all panned out??? I'm running a stock TPI intank pump which puts out from what I'm told about 35psi...I installed a Mallory 4309 hooked it all up and when I power up the pump my FPG cruises past 15psi and I shut it down so I don't wash out the cyclinders. Nothing I do brings the pressure down. I'm using the stock lines from my '86 conversion. The only thing I haven't done and that is talked about in this thread is check the return for a obstruction. Should I blow air through??? I used Summit -6AN to NPT fittings which they screw down in the hose pretty fair so I dont know for sure how to tell if there is rubber obstructing the line.

 

From what I've heard you can't use a high pressure (TPI) pump in a carb'd car, it'd overpower your regulator (at least at 12-14.5V). Try looking for an adjustable power supply to drop the voltage. I was going to replace the resistor with one of those, but now it looks like I'm going TBI.

 

The previous owner of this car actually was running his stock TPI pump for a while and was having issues. That's why he put this mallory 140 pump on. 15psi will more than likely overfill the bowls and cause fuel to spill over.

What I ended up doing was running a completely additional 3/8" return line. So, I have a 1/2" supply line from the mallory pump to the mallory regulator. Then I have two 3/8" lines running to the two carb bowls. But the return line I have a 1/2" line running out past the alternator and then I tee into two seperate 3/8" return lines. One is the original TPI supply line and the other ties into the original EVAP line going into the tank. That line is 5/16" at the tank but it still gives me enough return area to drop the fuel pressure. I can now turn it down all the way to 2psi while the pump is running flat out. And I can turn it up to 6.5-7psi where it'll sit all day long.
The reason I ran a second 3/8" line is because I didn't want to remove the original 3/8" and put a single 1/2" return line. My steel 1/2" supply line was hard enough to shape into place and flare that I didn't want to mess with that again. I bend the 3/8" line in two pieces. One goes from the engine compartment to just past the long tube header collectors, then it joins in with another section that continues on back to the tank.

It was some work but now I have rock solid fuel pressure.

If you turn the voltage down you'll also slow the pump down and lose flow. So if whatever method you use to turn it down leaves the pump at a constant speed then you could possibly run too low of pressure once the engine is under heavy load and high rpm as you start to run out of flow as the engine demands more.