Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
Surging in high gear
Cliff Ruggles:
Stock fuel pumps and fuel systems were NOT designed for 500hp, or anywhere close to that.
The truth is that most stock systems are hard pressed to keep up with much past about 350hp and we start to see all sorts of issues depending on the vehicle, tank location, routing of the factory lines, etc when folks install powerful engines and start taxing the fuel system harder. Heat can be a player here as well as lines often run too close to exhaust pipes and pick up too much heat on the suction side of the mechanical pump.
I went thru this with my own vehicle over 20 years ago and quickly learned that a stock fuel system on a 1973 Ventura is good for about 350hp with no issues anyplace.
When I installed my first 428 engine making about 400hp I didn't have any issues at all until I took steps to improve traction (DOT tires or slicks) and installed 3.42 gears and a good aftermarket converter to get the car to leave harder. The car ran fine in all scenarios until I found good traction and left hard, then it would start to loose power right up near the shift point (5500rpms). I could actually "feel" the engines power falling off around 5000rpms but it never ran completely out of fuel, most likely just sucking the fuel bowl pretty low.
I tried an HP Carter mechanical pump, pusher electric pump, and they really didn't correct the issues, the car still felt like it was loosing power past 5000rpms but only at the track or on the street when it hooked up really hard.
Next move was to install a more powerful 455 engine which INSTANTLY resulted in fuel delivery issues much more pronounced than with the 428 engine. The engine would pull like a freight train in low gear to about 4500-4800rpm's then nose over, quickly recover and pull right on up to the shift point.
It ONLY did this on hard runs with good traction. I could do "John Force" burnouts with zero issues, or blast thru the gears without any problems, etc. Any attempts to heat up the tires and launch hard and it would nose right over.
At this point I was perplexed by the problem and had tried several different things to correct it and each time I thought I had it beat then did a hard run with traction and the problems came right back.
The last attempt I did was to remove the gas tank and install a larger line and relocate the pick-up more to the rear of the tank as it was right in the front. I put a Holley Blue pump just outside the tank, and 3/8" steel lines to a regulator in front of the carb dead headed. Low and behold the problems still existed and I was completely stumped at this point. I even tried pushing to the Carter HP mechanical pump which wasn't quite as good as just re-routing around it.
.....continued in a second response as this one is beyond 3500 characters....
Cliff Ruggles:
continued......
The car by now was being drag raced couple of times a week weather permitting and running solid mid 12's with best ET of 12.43 at 109MPH. I settled on just short shifting the engine at 5000rpms for all runs right before where it would suck the fuel bowl low and start loosing power. At the suggestion of several well meaning folks I even built and installed a Holley 4781-2 850 DP carb and it didn't do much better, power still fell off at high rpm's. I would have thought that the much larger fuel bowls and 2 N/S assemblies would have certainly helped, but really no improvement as the car still ran mid 12's at 109mph.
So I just dealt with this problem for several years and enjoyed the car on the street and track, and logged over 1000 track runs with it without any noticeable problems. Over one Winter I decided to upgrade the fuel system one last time and pulled the tank and welded on a CE rear sump, upgraded to a Comp 140GPH pump, and 8AN lines/fittings everyplace.
First track outings the next Spring resulted in 12.0's at 112mph!!!!!!! I was simply amazed with the improvements and the engine pulled so hard past 5500rpms on the first two runs that I didn't get it shifted quick enough and actually ran a little slower than the runs to follow shifting at 5500rpm's. I ended up with a best run for the car that day of 12.02 at just under 113mph!
I would also mention here that I carried the big Holley 850 to the track that day and it also ran 12.0's at 112mph, showing that it was also not staying full on hard runs with the previous fuel delivery system.
Lessons learned in fuel delivery with these cars for sure. I went on to install a bypass system back to the tank to take some load off of the electric pump as I "fried" the brushes in the first one in about 2 months running it "dead head".
All of this happened in 1999-2000 and to date the current system has been FLAWLESs clear down to 7.18 @ 96mph in the 1/8th mile and nearly into the 10's in the 1/4 mile with a newer and more powerful 455 engine.
By this point in the long response folks have probably figured out that the pick-up location in the factory tank was the culprit and I was uncovering it on really hard runs. Installing a rear sump and moving the 140GPH pump behind the tank, and larger 8an lines everyplace also works for us on hard launches as fuel rushes back to the pump not being sucked away from it.
With this topic you will see a wide variety of threads on different websites where some folks run deep into the 10's with "stock" fuel systems where others can't get out of the 14's trying to do the same thing.
All I can say about all of that is that we have learned here to make sure the fuel system is more than adequate for the power level and vehicle performance and since those lessons nearly 20 years ago have had zero issues in that area.......Cliff
77cruiser:
Good write up Cliff.
trw:
Thanks Cliff.
Guess I will work on fuel delivery.
77cruiser:
I installed a 320lph in tank pump with bypass reg. & it's a lot better.
I wanted to stay mechanical but it wasn't working, I didn't want a sump & pump hanging down for the world to see.
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