Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Surging in high gear

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Cliff Ruggles:
One thing that I'll point out here is that folks are quick to toss out their Q-jet and go to some sort of aftermarket "high performance" carburetor thinking the Q-jet is simply not capable of keeping up with these powerful engines.

Folks quickly blame the small fuel bowl but in reality it simply let's you know that fuel delivery is inadequate for the power level much sooner than a carb with more fuel capacity.

I even see folks "chopping" up the insides of the q-jets fuel bowl, splash guard, etc in order to increase fuel bowl capacity when the real cure is to increase fuel delivery so it simply stays full on hard runs.

Of course you have to keep in mind that you only have ONE needle/seat assembly, so in addition to fuel delivery being adequate the size of the N/S plays a big role here as well.

What I've found over the years is that the myth of the q-jet not wanting or liking high fuel pressure has folks setting their regulators WAY too low.  The only q-jets that gave issues with high pressure were the early designs with the short hinge pin and huge float.  They can still be made to work for HP use but a later model is much more desirable.

Anyhow, what I've found is that a .135" seat is fine to at least 550hp and cars running well into the 10's IF you have adequate uninterrupted fuel delivery and at least 6lbs fuel pressure.

Of course that is just a blanket statement based on quite a bit of drag racing with good traction on half a dozen vehicles I've owned or sponsored out of the shop.

Even for those we seldom use a .135" seat and go to .145" instead, and run around 7.5psi.  This simply insures that fuel delivery is NOT an issue and if/when we have tuning issues we look someplace else.

There are also several different schools of thought on that deal.  I have several customer who run really quick and prefer smaller N/S assemblies and really high pressure, like using a .125" seat and 12-13psi with HUGE fuel lines and high flow electric pumps with a full bypass system.

Others doing the same thing will go .145 or even .149" and drop the pressure back some and claim perfect results.

For myself I've settled on simply installing a fuel delivery system capable of supporting at least 700hp even though my engine is a tad under 600hp.  This simply takes fuel delivery out of the equation and we have never once in nearly 2 decades now had the first hint of a problem at the track with good traction.......Cliff

73ss:
How did the factory high performance cars do with the stock fuel systems? For example, The dual quad hemi cars, tri power pontiacs & vettes, LS6 chevelles. They were all fed with a small looking pump. Did these cars have problems on the big end? Was there more to these fuel pumps than meets the eye?

I've had some problems with my set-up. feels like it's on the verge of running out much past anything over 5700. I did install a correct NOS AC-Delco pump on my 73 chevelle and It has been a big improvement over any other type of pump I've ran, electric, high flo holley, etc. I'm going to address the fuel system over the winter. Thanks for the write up Cliff..

Cliff Ruggles:
I don't they did anything to improve fuel delivery for those cars much beyond using a better pump on a few of them.

I owned and raced a 440 powered 1970 Roadrunner for quite a few years and it was fine with the stock fuel system until I installed 4.56 gear and slicks.  Then it immediately started pulling the fuel bowls pretty low on the big Holley I was using at that time and required upgrades to keep it full on hard runs.......Cliff

Hillbillyenginering:
I have used manifold vacuum pressure as an indicator of fuel shortage. It will be pulling hard vacuum maxing out and it will fluctuate. Been so bad the vacuum will start dropping all the way down and engine would die. That's with holding WOT.
HAD to eliminate the stock filter and installed 6na lines with filter.

73ss:
I did the 6 AN line from the pump to the carb with external filter a few years back and it got worse. The 6 AN line fittings have a tad smaller ID than the stock 3/8 metal line. I my case, The line may have been picking up extra heat.

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