Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: trw on September 25, 2017, 04:36:18 PM
-
Just rebuilt a 17080212 "800" quadrajet with Cliff's kit and recipe.
Pulls like a freight train in second.
In 4th it starts surging at about 4000-4500 rpms.
what did I do wrong
Thanks
Terry
-
How's the fuel supply? Is this at WOT?
-
Stock 73 Corvette fuel supply and yes at WOT.
No fuel pressure gauge.
The pump is only a couple of years old and worked with the stock Qjet.
It is a 489 with about 500 hp.
-
A stock mechanical pump would be hard pressed to keep up with that kind of power. Our kits contain the high flow N/S assembly unless you ordered a "basic" kit and did not specify that you needed the high flow seat.
With a .135" seat installed the carb will easily support 500hp if fuel delivery is adequate.......Cliff
-
I have the larger N/S.
How can I test to see if fuel delivery is adequate.
Would adding a small electric pump at the tank do any good? I have one laying around the garage.
Should I just get a high volume mechanical pump?
Thanks
Terry
-
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....
-
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
-
Good write up Cliff.
-
Thanks Cliff.
Guess I will work on fuel delivery.
-
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.
-
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
-
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..
-
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
-
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.
-
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.
-
I've heard many times of folks curing fuel delivery issues by removing the stock fuel filter and installing an in-line filter. The theory sounds great and it gets regurgitated frequently on many websites when the topic comes up related to using a Quadrajet in a "high performance" scenario.
So I took my car to a local test and tune session and did some testing.
When drag racing I used an 8AN line from the regulator (mounted at the front of the drivers side valve cover) to the carburetor. The fuel filter is a high flow racing type unit mounted behind the tank next to the 140gph fuel pump.
Never one single issue with fuel delivery with this system.
So to test the capabilities of a stock paper filter put a 3/8NPT to 3/8" steel fuel line adapter in the regulator and ran a 3/8" rubber hose to the carbs stock fuel filter housing with a paper filter inside.
I had already rigged this up and made two passes before installing it to replace the 8AN set-up.
I made three more track runs, two are shown below, the first one is before the change and the second one after the change 23 minutes later:
R/T: .505 .514
60': 1.6230 1.6289
ET: 7.3169 7.3188
MPH: 94.35 94.36
These are 1/8th mile ET and MPH. You'll notice that my daily driven street car is a little faster than most so fuel consumption on a run, and acceleration trying to put fuel back in the tank would be a little more taxing on the system than most set-ups.
I think this pretty much proves that fuel delivery to the nose of the carburetor is far more important than what filter you have in line on the pressure side of the system. As long as it's not clogged up the factory paper filter used in a Q-jet certainly appears to be more than adequate to support big HP levels and pretty quick vehicle performance......FWIW.....Cliff
PS: some folks may notice that it actually ran slightly quicker after the stock filter and smaller feed line from the regulator was installed. This is going to be due to a very slight improvement in weather conditions as it was done in the evening and things were cooling off pretty quickly that night.
-
I had always ran the stock filter. I have had multiple issues with my toro. Kept chasing my "carb" issue. In the process I found a bad distributor which next was the coil, replaced new MSD. Then wires. Rebuilt the carb with your kit. Kept having issues. On accel would fall on its face. Installed vacuum gauge and vacuum would max at 22". Then flutters and drop all the way to zero and die, with WOT. Letting off would recover. Figured out that the turbo 425 needs vacuum to stay in gear.
Installed a vacuum can to trans. Same issue with vacuum. Installed a fuel pressure gauge between pump and carb. Good at 4.6 to 6 psi. FINALLY figured out it was the filter. Yes it was new and in the correct way. Got so bad no fuel would go thorough. Torn it apart and could not see anything inside.
Of course I'm making the story shorter here.
I followed your directions and knew it couldn't be the carb. And after reading so many of these posts I just needed to keep looking.
It stumped me for so long because it was a new filter and never had one clogg up so quickly. Had to take a step back and ask myself. If this was somebody else's car what would you do.
-
I would say that fuel delivery is marginal right to start with if simply removing a stock filter and putting something else in line helped.
My post above simply shows that when fuel delivery is adequate for the power level the stock filter is NOT restrictive enough to cause any issues.
Even with that said just about every vehicle out there is different for fuel delivery as far as the configuration of parts. Factory sumps are poorly located and easily uncovered on hard runs, and the "sock" on them can become quite restrictive over many years of laying in the bottom of the tank. Stock lines have many bends and often get close to heat sources and are also subjected to a lot of heat as they snake their way up to the stock fuel pumps.
Mechanical pumps, even really good ones are still not nearly as good as electric pumps for fuel delivery. They have valves in them that open/close so the fuel flow stops/starts with each pump stroke. They still make really good high flow mechanical pumps, however my testing with them has shown that unless they are fed with HUGE fuel lines and unrestricted all the way back to the tank they can't reach their full potential.
It is also ALWAYS better to "push" all the fuel than to "pull" it thru nearly 14' of fuel lines for a "high performance" application......FWIW....
-
I had always ran the stock filter. I have had multiple issues with my toro. Kept chasing my "carb" issue. In the process I found a bad distributor which next was the coil, replaced new MSD. Then wires. Rebuilt the carb with your kit. Kept having issues. On accel would fall on its face. Installed vacuum gauge and vacuum would max at 22". Then flutters and drop all the way to zero and die, with WOT. Letting off would recover. Figured out that the turbo 425 needs vacuum to stay in gear.
Installed a vacuum can to trans. Same issue with vacuum. Installed a fuel pressure gauge between pump and carb. Good at 4.6 to 6 psi. FINALLY figured out it was the filter. Yes it was new and in the correct way. Got so bad no fuel would go thorough. Torn it apart and could not see anything inside.
Of course I'm making the story shorter here.
I followed your directions and knew it couldn't be the carb. And after reading so many of these posts I just needed to keep looking.
It stumped me for so long because it was a new filter and never had one clogg up so quickly. Had to take a step back and ask myself. If this was somebody else's car what would you do.
You probably had a filter with the check valve? That had me stumped a few years back
-
Yep. Had a check valve. But what confused me was how clogged it got. Would not pass any fuel through it!
And taking it apart did not show any evidence of contamination.
-
Was reading through this old post and I am having the exact issue that Cliff has described. I made some changes and finally got the car to start hooking, (1.8 60', not the greatest, but a small victory for me) only to have the ET and MPH slow down. Car noses over at 3500~4000 from 1st to second. The car has always been on the edge of running out of fuel. I can't get to excited about sumping the tank. I've been looking at some of the aeromotive in tank kits. Anyone have any experience with one of these? Any other ideas?