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quadrajet: the fuel pump, fuel bowl, and accelerator pump

my kids took the truck

my kids took the truck

1/2 ton status
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Oroville, CA
Summary
A thread for the fuel pump and quadrajet combination --> overtime the thread increasingly focused on just the performance of the accelerator pump related to the fuel bowl volume after sitting for a few days. Meaning, it won't start like it should - I want to know why.

Read
I am reading through "The Design Evolution of the Quadrajet" written in 1966 by the designer of the q-jet Donald Stoltman. In the paper he explains that the small bowl design is intended to improve the angle before fuel spills out the nozzle and to prevent fuel evaporation since the dwell time in the hot carb is reduced. He relays that due to the small bowl design the fuel pump flow and maintaining pressure within +/- 2psi of design pressure is critical to performance.

There is a table here (I will add the image), it is difficult to understand but it looks like:

  • Due to the needle seat design, for a 300HP engine fuel flow can be as low as 1 psi to supply 2.5 pounds of fuel per minute (ppm). I converted 2.5 ppm to gpm, it is .4 gpm ~ 24 gph at 72 deg F, not sure what it is at 100 deg+.
  • 8 psi is required to maintain a constant fuel level, and fuel pressure should be maintained between 6 and 10 psi. I don't see how this 8psi relates to the 1 psi sufficient for 300 HP. I am guessing it must be volume/pressure related.
  • By design, once the fuel level is over .25" from zero, it will 'blow-off'
Obvious Problem
My K5 is backflowing fuel. I can see fuel backflowing through the clear fuel filter after I turn it off. The fuel pump seems to work fine, so the check valves must be good enough. But, as I understand it, the fuel should not backflow.

Intuitive Solution
So, I can buy a backflow valve and put it inline, or replace the fuel pump. If I replace the fuel pump, then getting the right flow and pressure seems important. Or, is a separate fuel pressure regulator a good idea?

Question
What works on your quadrajet?
 
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I found my backup fuel pump from the local counter parts store, Carter M6624 40gph.

Inlet Nipple Size: 3/8"
Outlet Fitting Size: 1/2" Threaded --> I think this is wrong, it is a 5/8-18 in. Inverted Flare

What this pump exactly specs to is debatable.

They all seem to agree it spec at 7.5 ~ 9 psi.

I've had this one in the tool box in case the 20 year old one goes.

Over at smokemup.com they discuss this pump but they thought it was a 120gpm - they calculated that a sbc 350 in the bast case at 6500 rpm needs 42 gph; just over what this pump does, but I don't go 6,500 rpm and it is not the best case 100% VE.

Is this style fuel pump good enough?
 
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What should the fuel pressure be assuming 40 gph is sufficient.

I don't understand the technical report requirement for 8 psi if 1 psi is sufficient to open the needle valve. As long as the flow is 40 gph the pressure should be irrelevant. I assume the 8 psi is necessary to flow 40 gph through the valve orifice. But then that brings in the question of why they thought 1 psi is sufficient. The only explanation is you need 8 psi to flow 40 gph through the valve seat. But, in the event of a fuel pressure failure due to vapor problems caused by heat, the valve will still flow down to 1 psi (whatever the gph flow through the orifice is at 1 psi, most likely not 40 gph) therefore preventing vapor lock.

But, in real world driving what quadrajet psi is best:

  • hotrodders.com has 5psi
  • another hotrodders.com with actual example, 5psi
  • smicarburetor.com
    Carburetors love fuel volume, but hate pressure. Pressure creates inconsistent aeration of the fuel in the float bowl, which causes inconsistent metering. Picture in your mind a water nozzle spraying into a bucket; the more pressure used, the more froth and air bubbles are created. If you have proper volume, the optimal fuel pressure is 4PSI for modern 2 and 4 BBL carbs.
    Click to expand...
  • aquacomp.net 5 ~ 6 psi, most posters are using a fuel regulator to maintain pressure
  • chevelles.com 4.5 ~ 5 psi with regulators
While these all use 5psi they don't say what size fuel line or material type they are using, the seat size, or the filter back pressure.
 
I found a orifice flow calculator, this is for water so I assume gasoline will flow a bit faster

http://www.tlv.com/global/TI/calculator/water-flow-rate-through-orifice.html
this site time you out and basically wont let you use it again for half an hour, I had to grab another laptop

For each orifice size I will give at 8 psi and 5 psi, starting with 0.180, the original design seat size, the biggest I see for sale now is .150

in gph

.180 102.5 80.9
.150 71.3 56.2
.145 66.7 52.5
.135 57.7 45.5
.125 49.5 39.0 --> seems to flow the necessary 40gph @ 8 & 5psi
.110 38.3 30.2

For the fuel line, what psi is needed to flow 40 gph?

I found a calculator but it does not give us exactly the answer
http://www.gates.com/catalogs-and-r...s/fluid-flow-calculator/fluid-flow-calculator

At 40 gph

5/16" has a 1 psi drop --> so an 8psi pump will deliver fuel at 7 psi, too high

3/8" has a .6 psi drop --> fuel delivered at 7.4 psi, too high

This assumes the suction side of the pump does not reduce the pressure on the output side, I doubt this is true. But I have not seen how to calc the drawdown.

Someone give me a reality check on these numbers. It seems like my 8psi fuel pump needs a pressure regulator or I need a different pump. Can you change the springs on these to change the pressure?

Talked with my father-in-law, he keeps piss-beer flowing at the ol' brewery, he suggested that I need to calculate the pressure loss due to the head between the bottom of the engine where the pump is located (and top of fuel tank) to the carburetor at the top of the engine.
 
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Back in the day some engines that used mechanical fuel pumps ,you could "adjust" the fuel pressure (as in reduce it) by adding more gaskets between the pump and its mounting surface,which reduced the stroke on the push rod or cam lobe that operated it...

Your REALLY getting into carbueration,aren't you?...:eek1:
 
quadrajet and the fuel pump

Problem with most mechanical pumps today is excessive press, hence regulators being used quite often as you have mentioned.
Later quads seem to run fine on 7-8 psi but the problem comes in after shut down hot. Expansion from heat causing that pressure to rise higher and blowing off the needle/seat while sitting and what you have is a hot start problem.
Pedal to the floor to get it to start.
From my experience most after market pumps put out too much pressure causing multiple problems.
I've had flooding problems, hot start problems etc from too much fuel press.
Which reminds me about your drain back comment, that's not necessarily a bad thing during hot soak situations.
Fuel pressure increase due to heat and a fuel pump that will not relieve pressure is a recipe for the above, been there.

A regulator or a different pump is the only solution.
I use a edelbrock street pump 6 psi max and allows the press to drop off after shutdown which eliminated my issues with high fuel press and hot start problems.

From my understanding some use a stock new a/c delco pump that maintains correct pressure.
 
Also regarding fuel pumps and press today's fuels with ethanol Creates hard starts and increased fuel boil off and evaporation due to volitlity.
 
adding a pressure regulator to the fuel system - might as well fix the rest

can someone give me a check on this setup:

A) OEM style fuel pump --> 8psi

B) Fitting, 90 Degree, -6 AN Hose to 5/8-18 in. Inverted Flare

C) 3 foot PTFE Hose

EDIT: backflow checkvalve? --> I removed the 'hook' from the float and the backflow stopped.

D) -6 AN fuel filter

E) -6 AN pressure regulator

F) Fitting, 90 Degree, -6 AN Hose to 5/8-18 in. Inverted Flare

G) 5 psi --> quadrajet fuel inlet

the quadrajet will have a solid fixture of fuel_filter+pressure_regulator+6AN_to_5/8IF adapter attached to it. It would probably be better if I fabricate a brace to support the weight, rather than just it hang off the inlet threads?

The cost to change from you-name-it parts hose to 3' of PTFE is not much

nice PTFE walkthrough explanation over at dmstuners.com

I like the PTFE Hose for the fire safety. It complements the roll over shutoff I installed on my fuel tank vent. Hopefully, it is never needed, but in the event of a fire, I'd like the fuel line to stay intact. And, now is as good as any to start thinking about accommodating higher concentrations of ethanol.
 
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I looked around the web for a few nice photos to help see what has worked

Over at 67-72chevytrucks.com I found this clean setup posted by SPORT/TRUCK. I looked through quite a few images online and this was the only I found with a quadrajet. It is a nice clean setup, and by luck it is a sbc and a truck.
DSC02484-1.jpg
 
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it looks like regulators are either 1 ~ 4 psi or 4.5 ~9 psi. The recommended quadrajet pressure on most threads is 5 psi. That is basically between the range of these two. Is there a 1psi ~ 6psi pressure regulator?
 
Is a backflow preventing device a good idea?

With sealed well plugs, there should be plenty left in the float bowl for a start even if the needle was off the seat as I recall...it's a good 1/2" plus from the bottom of the bowl.

As mentioned, if the fuel heats up, I would expect it can't go anywhere but vent upwards, which would spill it into the venturi's from my experience. Had a mal-adjusted float, and at idle it would overfill the carb, pour into the venturi's, and cause rough running and stalling.

I can't recall which book I had (probably an old hard back Chiltons? It also listed the rated HP/TQ figures from everything from the late 60's up to maybe the late 70's) but I do recall that it listed the GM fuel pressure ratings. No clue where those figures had come from, but I recall that the GM stuff was "all over the map", indicating the Q-jet could tolerate a fair bit of variation when set up properly.
 
dyeager535, thank you, good stuff to check

I think I am going to abandon the idea of fuel regulator. With the OEM-style pump, 3/8" fuel line and in-line filter, the pressure at the carb seems ok. I will put an inline gauge to verify (finally did this here), but I am betting it is around 6 psi. If I change anything, then, the regulator might be an option again.

I don't want to start comparing holley versus quadrajet, but having ran both of them, and having now messed with both a fair bit, I am starting to see the #1 issue with quadrajet is fuel bowl related. On the holley, it was fuel metering. Looking at the design of the quadrajet metering rods and the huge accelerator pump, I can see why these are basically no problem. The holley has really nice fuel bowls that are easy to adjust, don't evaporate, and don't maybe/sortof/don'tknow leak out of plugs. Then there is the tilt capacity of each design. Quadrajet wins out of the box. But, it is almost too good. I'd never drive at a 45 deg angle in any direction. And, with some mods, the holley seems capable of high angle, I don't know if it is 45 deg but it is enough to get you nervous. So, I got it out on holley v's quadrajet.

I am certainly missing fuel. Even after a day the single pump on the gas and ignition requires a couple trys. This seems a trait of the quadrajet. Years ago, the last time I ran a quadrajet, I just thought my truck was old.

This time I will solve it.

My idle is super nice. Never stalls. Even at cold startup it just needs a few minutes of fast idle at 700rpm to hold idle speed at 600 rpm without fast idle. But, once it warms up, the idle climbs to 800 rpm. And stays there. I think this is warm fuel evaporating from the bowl and richening the intake air.

My first change is to use the fast idle cam to hold it at 700rpm for warmup. Then adjust the hot idle to 600rpm and expect the evaporated fuel to richen to mixture. Right now, I have one idle setting for cold and hot. I can't find that happy median where both work well.

Next, I am going to keep the fuel cold. With heat wrap on the fuel lines and heat shields at the headers/fuel_line crossing and fuel_pump/headers, and at the fuel tank and exhaust pipe interface.

And, right now I have a plastic 'button' style plug in the power-valve adjuster hole. It keeps the fuel from spilling out and keeps dirt out. This is probably letting fuel vapors pass. So I will tap and plug the hole. Though I don't see how it is any different then the vents.

And last, against some good advice. I am putting a wood spacer under the carburetor. This should reduce the carburetor bowl temperature. With an attempt to keep the characteristics the same: A closed 4-hole spacer to keep the plenum volume close to the same. And then I am going to open up one side of the spacers primary holes into a 'tapered' style opening to unshroud the butterflys and allow part throttle airflow to pass at an angle without hitting the spacer wall.

EDIT: Also, just ordered a rebuild kit form Cliff's. The rumor is the NAPA kit I used is junk. So, my accelerator pump skirt might be leaking already. And, I liked the idea of playing around with the power-piston springs. I guess the NAPA kit was just to see if I liked how the carb worked. I want this carb to start with one push of the gas pedal and as soon as I touch the ignition (my holley does it everytime on this truck) - this turning over stuff needs to stop. As the Marines say, "this recruit is going to kill this one"
 
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Wouldn't the OEM paper fuel filter with the "roll over valve" act as a check valve to prevent back flow?...
 
yes, but I don't use one of those - I have the old school glass filter. It is the one nostalgic semi-functional piece I keep on my truck. I guess it is like fluffy dice on the mirror. It is dumb but it conveys the era the truck was built.

I don't want to double up on the filters. Plus, you can't see if the filter is clean. And opening the q-jet filter housing is a pain. It leaks unless it is tightened just right. And, the threads strips if it is overdone.

Consensus seems to be the check valve is not necessary based on the bowl design.

But, if it takes that. Then the glass filter is going in the box with the holley and I'll put the OEM-stlye filter in.

EDIT: nice score on the diesel suburban!
 
My opinion anymore is that insulating fuel lines is a BAD idea.

Now granted, my TPI setup is likely to flow a fair bit more fuel (although I think it runs a 5/16" output on the pump vs. the 3/8 lines GM used on everything else) but after my problem with a split fuel hose that dropped pressure to ~24PSI, and some research, I believe the lines are effective radiators of heat.

I actually measured the fuel rail temp (should do an after test, but I know I fixed it) and the fuel rail, with low pressure, was absorbing a LOT of heat...to the point that the rail temp was nearly what the intake temp at thermostat was. I found others were complaining of hot fuel in the same setups, and that the fuel in the tank was actually getting hot. My presumption is that the return lines/flow allows heat picked up near the engine to be dumped via the return line, and on the feed side prior to getting to the engine compartment. Shielding them near the engine/headers will likely help, but you have a bunch of temp variables to consider further back. IMO high volume, just like a cooling system, would be of value in that instance. With the drop in pressure the fuel wasn't flowing through the rails in enough volume to keep from absorbing too much heat. Since not as much was being returned, the fuel that did return would be hotter than normal, starting a nasty cycle.

Obviously GM knew this was an issue since they started using return lines. No other reason to do so.

Dad had similar issue with his truck when carbed, tried wrapping the fuel lines, but didn't seem to make a difference even in the engine bay. I can't recall what the solution was though. :(

I don't recall ever running into an issue where my idle climbed. If anything I'd make very certain the well plugs aren't leaking. If it was bad enough, I'd think heat would cause even more leaking. If the float bowl goes dry overnight then I'd be looking at those first. Pretty easy epoxy fix.

Edit: Not all of the GM filters had the "check ball" in them I'm certain. I'v taken apart my fair share of Q-jets, only seems like a select few ever had that as stock.
 
I think you just nailed it. GM engineers knew they had a fuel temp problem and the bean counters would not let them solve it. I can assure you that a box of heat shield material is not cheap. And installing this stuff is not easy. When pennies count. I am certain the bean counters decided it runs good enough without heat shield for most applications.

All that said, why is it a bad idea to shield the fuel lines?

One concern I have is just shielding around the engine block - what I have now - since the fuel probably picked up the heat at the exhaust headers and now the shield is keeping the heat in.

Another is corrosion. If the heat shield is a wrap then moisture/mud/snow will get in there and rust the lines quick. I have a box of material and still trying to figure out a design that shields the heat, reasonably easy to install, and prevents trapping moisture and dirt. I am thinking about a sheetmetal shield the length of the exhaust pipe. And then a more expensive flex shield at the axle bump near the fuel tank and at the headers. Or form and attach a sheetmetal shield to the tank corners by the exhaust.
 
My opinion is that if you shield the lines, you are also insulating them. If there is a return line to make sure the fuel in lines near the engine doesn't get too hot, where would the heat absorbed be sinked except through the fuel lines?

You can dump the fuel into the tank and let the tank fuel absorb that heat, but as fuel levels decline, engine run time is increased, and ambient temps increase, you are likely to start having issues related to using the tank solely as the sink...the fuel in the tank isn't really "moving" effectively without a constant flow of fuel in/out, and I'd argue that based on the volume in the tank, creating effective flow within the tank to constantly move fuel from contact with the fuel walls would be somewhat ineffective. When fuel moves through the lines, it is going to be in far more contact with the exterior wall of that tubing than the tank walls.

If you were using that shielding, I'd think almost a "wall" over the open section of the frame c-channel would be the preferred choice, as you say if the lines get warm, then condensation/rust is going to be an issue over time if the lines are wrapped in something that traps moisture.

I can't think of one application (and I have limited view on this I admit) that GM used heat shielding on the lines. There must have been a reason. They went so far as to install fans that blew on the carbs on shut down in some applications, they must have felt shielding was not effective, and in the case of having to run additional electrical components vs. some non-failing insulation, that would seem to be a logical conclusion even regarding cost.
 
My '74 K5 does not have a return line. Same for the '72 1500 I used to have. I see what you are saying now. Without having spent much thinking about return, I did not understand your point.

If I understand. The fuel tank is the heat sink and the return line pulls the heat away from the engine compartment located fuel system. But, this is only partially effective since eventually the sink becomes heat saturated and then you have a vicious feedback loop gaining heat. And, even with this, they never used exhaust heat shields around fuel lines and tried more expensive/less reliable solutions. So we can assume, they tried heat shields and they did not work. Meaning the significant heat source is the proximity to the intake manifold. Even if the exhaust heat is removed, the fuel will heat up anyways at the intake.

With that lesson learned. I am going to think about this for a bit.

My instinct is to insulate the carb. Cliff Performance warned against this. He said it will cause other problems. I ran a 1/2" 4-hole heat insulating spacer under my Holley. Open headers, no heat riser to move exhaust through the intake manifold crossover. And drove it daily in the Sierra Nevadas with an open air cleaner. Down to 15 deg F. Sometimes the 180 deg thermostat wasn't even open, the engine was maybe 165 deg. Plenty of whiteout conditions. Never had a problem starting, idle, or with drivability. It was better if anything.

I checked online to see if there is an ethanol-free station nearby. No luck.
 
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