fuel pressure question
 

H/SA 350 sbc with quadrajet(Deano carb). Car has a Magnafuel Prostart 500 fuel pump(rated 26-38psi) 12an line to front, Product Engineering Regulator rated 4-12 psi. I am pretty sure that the carb is letting fuel by the needle and seat. It is very rich, sometimes floods out, but then other times at an idle it is perfect. I can not get pressure to go below 6 psi. What is too much pressure for that carb? I know the pump is overkill but it was on the car when I got it. I tried a barry grant regulator that I took off a holley black pump system that was set at 7 psi, it went over 12 psi when I turned the pump on?? SO... do I have a carb issue, too much fuel pressure, or Both lol

Re: fuel pressure question
 

The Holley "Black" pump would make more sense with the BG regulator (providing it is still working properly). The volume is sufficient (12 an) to the regulator and reduced to 8 an to the carburetor. Always flush the fuel line prior to hook up. The needle and seat is an area that "must" be clean at all times. The float setting of a 1/4" down is a good starting point also.

Re: fuel pressure question
 

What needle and seat size do you have?

With a 0.125 seat you should be able to go as high as 8 PSI. With a 0.135 seat or larger, I stay around 6 to 6.5 PSI. Check for seat and needle assembly wear or dirt. With the hard to find Grose-Jet needle/seat assembly, I have gone as high as 12 PSI.

Re: fuel pressure question
 

Could be too much pressure from the pump feed the regulator . Try reducing it down to 15 psi . Also I have had problems with debris getting caught under the regulating ball inside the regulator , pressure to the carb goes way up . I can usually recover by shutting off the pump while the engine is running then after most of the fuel gets used up in the carb , turn the pump back on , and usually the flow will flush out the debris .

Re: fuel pressure question
 

Pump will not go lower than 26psi. Thats the lowest it can be internally regulated. I really think it is too much pump and causing spikes at the guage that I can't see(gauge under hood)while car is under a load. As fuel demand goes up I think the internal bypass is causing it to just push by the regulator. I can take the adjustment screw on either regulator all the way out, in my hand, and it still has the same issues. I also just discovered that there is NO fuel filter in this system anywhere

Re: fuel pressure question
 

Sounds like the regulator is creeping pressure (leaking by the internals). Bummer but no surprises there.:rolleyes: Maybe you should proof your regulator's condition.

A very good rule of thumb on QJs is to use 5.5 to 6.5psi and that will fit with all N&S sizes.:D

Regards,
HB2:)
Dissident

Re: fuel pressure question
 

There has to be some sort of regulator in the fuel pump . Just like a Holley Blue which has a spring that can be shimmed or cut down . Does your pump have a bypass that gets plumbed back to the fuel cell ?

Re: fuel pressure question
 

Yes, I have adjusted it as far as I can. This is off their website

This pump, like all MagnaFuel pumps, is compatible with all racing gasolines and alcohol
fuels. Fuel additives are not recommended and may void the warranty. The MP-4401 and MP4450 have and external by-pass is adjustable from 25 to 35 psi. They come preset at 28 psi.
The PRO STAR 500 Series has enough flow capacity to support 2000 HP on gasoline or 1200
HP on alcohol. All MagnaFuel pumps require a #8 return line and a #8 vent or rollover valve.
Return line and vent must be above the fuel level. The Pro Star 500 series pump is designed to
work with a dead-head style regulator

Re: fuel pressure question
 

Had the exact same problem on 2 cars. Installed a -6 return and problems solved. Went this way in a street car that had a -4 and a car with no return. Solved both cars right down to rock solid 4 lbs. One with quadrajet and one with 700 Holley

Re: fuel pressure question
 

It might be a good idea to stick a big fuel filter on the outlet side of the pump . Reduce the fuel pumps external regulator pressure down to around 15 psi by cutting down the regulator spring a bit at a time .

Re: fuel pressure question
 

It has an 8an return line from pump to cell. It is plumbed exactly like magnafuel recommends. I'm gonna put a smaller pump on and see what happens. This pump supports 2000hp lol....

Re: fuel pressure question
 

Sorry didn't read that on original post. I have heard some fast cars using the same size return. -12 in and -12 out.
Also on my Olds wagon I'm Magnafuel 400 with -10 and their regular with no return and only get or down to 7psi. The -6 let me get down to 3 psi but I I raised it to 4.5. I was able to run regular screw out about 4 revolutions after making change.

Re: fuel pressure question
 

I had a similar issue with my ProStar 500 pushing thru Holley regs. Magnafuel sells a spring to reduce pressure along with opening up the return orifice to 3/8" helps. Using a Magnafuel reg. for the carb also helped as it would still push thru a little on the Holley regs.

Re: fuel pressure question
 

I had this same problem with 40 micron filter inline before pump. The new regulators are extremely sensitive, mine is a Magnafuel. They said need another filter before regulator, 10 micron I believe but can find info on their site. Problem solved. If you don't have any filter this very well could be your problem.

Re: fuel pressure question
 

I would suggest buying the total system from one manufacture , pump-filters-regulator. lines and fittings. As for line size, follow their recommendations , supply and return. I don't think you want to try to dead head this system. I installed an Aeromotive system last year. from the fuel cell to the carbs(2). Yes it is pricey, but it works!!

Re: fuel pressure question
 

Something to consider when planning or applying a fuel system of mix and match components. Mismatched components and dirty fuel systems cause a great percentage of the problems folks have to work on. All fuel systems are dirty whether with lines cuttings or old line deterioration. Taking the time and caution for filtering the fuel put into the tank through something so simple as a coffee filter is a natural solution.....cheap and effective.:cool:

Fuel systems should be evaluated as a SYSTEM where all the components must work together and that basic thought will get you very close to a best performance package!:D

The "old tried and true" (sic) Holley regulators will not hold more than about 21-22psi line pressure and at even less than that they have significant pressure creep problems. They also have the greatest "droop" curve when tested. Those things were designed to operate with a 10-12psi system pressure back in the day.The only saving grace is that they are cheap....:rolleyes:

A reminder: What you need is flow at pressure where the system pressure before the regulator is opposing the gravity of the vehicle's acceleration. That is if the fuel is in the rear of the vehicle and the carburetor is in the front. It is not the expense of the components that qualify them, but their actual functions that make the best selections.:eek:

Regards to All that like this kind of stuff,
HB2:)
Dissident

Re: fuel pressure question
 

I’ve been through this twice. Answer is a -12 return line with a return style regulator to lower the pressure enough.

Re: fuel pressure question
 

When it was said the bypass system can be adjusted so that the pressure is low enough that meant as restricted by whatever the carburetro needle(s) and seat(s) will hold.;)
I hope I have understood the premise and have tried to attach a sketch in respect of the physics of drag racing.
Rgds,
HB2:)
Dissident

Re: fuel pressure question
 

Well, my sketch that I thought would describe the fuel system situation very well is way too big :o (some 20K characters vs 600 allowed), so perhaps this is a better way to present some logic about the subject

See posting below for website link :DThat website is worthwhile to learn something about a method how trustworthy firemen train to overcome gravity. Keep in mind that water has a head pressure of 0.43psi per foot while a typical gasoline has a head pressure of 0.33psi per foot.
Some people want to learn how to apply some sound physics to their racer and others don't.:cool: Most who study the application of physics and sound engineering principles end up with faster racecars and bigger smiles.
https://www.fireengineering.com/wp-c...icles/document


Regards to All that like this kind of stuff,
HB2:)
Dissident

Re: fuel pressure question
 

https://www.fireengineering.com/wp-c...rgeonOct12.pdf
That is the whole link.....Guess I am going to have to have my local grandson school me on how to work postings on this forum. LOL
Enjoy,
HB2:)
Dissident

Re: fuel pressure question
 

In my experience, the PE dead head regulators like a "bleed" to minimize the droop when you go from low demand (idle/2-step) to high demand (launch, etc.). Too low of input pressure from the pump will cause regulated pressure to creep. You want to maintain at least double your intended regulator pressure at the regulator inlet during high demand. Have you been able to check the pump pressure?

The PE bleed is a jetted orifice that allows the dead head regulator to always bypass fuel but still creates a system with higher pump pressure than regulated carb pressure. A simpler version of this used to be common: https://product-engr.net/collections...ulator-systems

True bypass systems such as many engineered by Aeromotive operate at the regulated carburetor pressure throughout the system.

Re: fuel pressure question
 

By definition, a regulator is not a dead head device as that would be a bubble tight valve. The regulator must be flowing some amount in order to be set correctly (see below).;)

By definition, a droop curve for a regulator is a plot of pressure vs flow. The weep system that you described is for A) controlling the creep of the regulator and B) has the regulator already regulating so it has a much more rapid response in applications such as N2O or other.;)

If I knew how to attach a plot of a series of various regulator droop curves I would do so, but I am at a loss on how to accomplish doing that with a big file.:confused: Sufficient to say that some have drastically different curves.

Also to a previous post: Holley carbs with two needles and seats of 0.110" diameter have a working bowl volume of 0.066gal and a QJ has a working bowl volume of 0.022gal. The N&S of the QJ would have to be about 0.155" diameter to flow the same as the Holley as listed above......Not practical. 0.150" is common, but larger is pretty iffy.:D

Regards to All that like this kind of stuff,
HB2:)
Dissident

Re: fuel pressure question
 

I think to bowl volume of my beloved Thermoquads is close to the Holley Dualfeeds?

Re: fuel pressure question
 

I used Aeromotive pump and Magnafuel regulator -10an return and vent and feed to regulator

Re: fuel pressure question
 

So far looks like it was the N/S in the carb, took it apart and cleaned everything. Tried the reg off our other stocker and it went right to 6.5. Put my Product Eng reg back on that was set as low as I could get it to 6. It was then at 3. So I was able to screw it in to get 6.5. Much better. No more trying to crank like the timing is too high lol. It cranks from outside the car now. Now to change the oil and hope my brand new total seal rings didn't get washed down....oh and fixed my cracked 200 that lost second gear and cost me a round win. Get you a Stocker....it'll be fun they said