Qjet tuning tips

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Well-known member
Sep 26, 2009
Winnipeg, Canada
Tuning for Performance with your Quadrajet

(these are old notes that I obtained through my contact Dorian Yeager - they work! Enjoy - Corsair)


Here for your digestion are some easy mods, with brief explanations, that you can make to any Quadrajet. They require only simple tools, and most are free.

Setting the Air Valve (AV) tension
The AV is the flap that covers the secondaries of your carb. It opens up when airflow through the secondaries demands it. Decreasing tension on it has 2 main benefits. It allows the AV to open faster, giving you full power sooner. It also allows the AV to open more easily with airflow, giving you the possibility to make more total power. It is held shut by 2 things.

First by the choke pull-off diaphragm (which is on the front pass. side of the carb.) When the engine is running , the choke pull off keeps the AV closed by pulling the linkage via the rod that connects them. When you go to/near WOT, the choke pull-off pulls out (due to the loss of manifold vacuum) and allows the AV to open. The pull-off generally releases slowly, slow enough that even with no AV spring tension you shouldn't get a bog. The rate at which it pulls out is not adjustable with the metal can pull offs our carbs came with.

The AV is also held shut by spring tension, and that is adjustable. To adjust it, you will need a small allen key (either 3/32 or 5/32, i forget) and a
small flat screwdriver. (**note: it is possible that instead of an allen key you will need a small torx bit, but they are rare.) Now, here you go:

1.Look at the pass. side of the carb (with the air cleaner off)

2.Observe the linkage on the side of the AV. Through the slot you can see a small flat screw. That's the one you need to adjust.

3.To adjust it you need to loosen the setscrew, which is on the underside of the lip of the airhorn. The setscrew is where the allen/torx bit comes into play. Loosen it only enough that you can turn the adjusting screw.

4.Turn the adjusting screw counter-clockwise until the AV flops open on its own. Now slowly turn it clockwise until the AV just shuts. That is 0 tension. If you were setting it to factory specs you would then proceed to turn it down the specified turns (usually 1/2-1 turn). Instead, tighten
the setscrew and go drive.(**note: You may have to hold the AV rod to the pull-off out of the way while you adjust the tension. You can also remove the pull-off, but that is really excessive. If you exercise some common sense, it isn't hard to figure out how to hold everything.)

5.With your car fully warmed up, come to a stop and then nail it. Did it bog? If it didn't, consider yourself done. If it did, then there are a few ways to proceed. First ask yourself if you have a fresh tune-up. If not, what are you doing making performance mods when your basics
aren't even done. Shame on you. Also, be sure your choke pull-off is working properly. If it is bad, it could cause you problems too. Second, now is a good time to read the next section on changing secondary metering. Finally, if you don't want to change secondary metering (and
why not?), you can increase the AV tension until the bog goes away. Go in 1/8 turn increments at a time, and stop when the bog goes away. Do not increase the tension to over 1 turn down or you will permanently distort the spring.

Changing the Secondary Metering Rods and Hangers.

1.Look at the top of the carb between the flaps of the AV. Observe the little screw on the hanger.

2.Take that screw out. Most likely you will not own a torx bit small enough to remove it. The easiest way to take it out in that case is using pliers to turn it out. It sounds barbaric, but it works. If you didn't have the proper torx bit, while you have it out, modify the screw for future use. Either hacksaw a slot in its head to accommodate a flat head screwdriver, or file its sides flat so that it is easier to grip with pliers.

3.Lift out the hanger and the rods with it. REMEMBER WHICH WAY THE RODS ARE ORIENTED ON THE HANGER.

4.Swap the rods or hanger or both with what you want to drop in.

5.Drop them back in. They should drop in with no trouble. If there is friction, you probably have the rods on the hanger backwards. Don't start bending the hanger to make it fit until you try reversing the way the rods tips are facing.

Tuning Notes
Here are the basics of rods and hangers. The thinner the tip of the rod, the richer it is (more fuel can flow around it.) The thin part at the tip is called the power tip. The longer it is, the faster you go under enrichment when you floor it. For drag racing purposes, and in general, a longer
power tip will give better response and performance. Rods have identifying letter stamped on their sides. The letters are nearly meaningless though without a book on rod specifications. Instead, compare them by sight. If you do have access to specifications, look for thin tips and long power tips (also found by short tapered sections). A great all around set of rods IMHO are the CKs, which are marginally richer than stock L69 rod, but with a longer power tip.

Hangers are identified by the letter stamped into the top of it, the lower the letter, the higher it holds the rods out of the jets, giving you more enrichment sooner. This can be especially helpful in covering a bog going into full throttle, as well as for some extra grunt coming out of the hole.

You can find replacement rods and hangers at a few sources. Edelbrock and GM both stock new units. You could also try poking around in boneyards and swap meets. New, rods should only cost you about $8, hangers about $4. Edelbrock parts can be easily ordered through Jegs
(including CK, CC, and CE rods, as well as a good assortment of hangers.)

Increasing Airflow
Stock our carbs are limited to about 600cfm total airflow due to a tab that prevents the AV from opening fully. However, our throttle bores will support 750 cfm, once the AV is allowed to open fully. If you've got a reasonably well-modded car and you think more airflow is in order, here's
how to fix that.

1.Look at the AV linkage. Rotate the AV open and observe the little tab on the bottom of the linkage that hits the underside of the airhorn lip.

2.Cut, file or otherwise mutilate that tab off. (*see note*)

3.If you want to get really trick, you could drill a hole in the airhorn casting where the tab hit and put a setscrew to adjust the total opening of the AV. This could be used as a tuning tool, along with different metering rods to change airflow and fueling to suit current conditions.

(*note: When removing the tab, be careful how much you remove. If you file it down flat, you will definitely get interference with the AV linkage. When that happens, you have to get creative and tweak the rod and/or cut the slot in the linkage longer to allow it to work. If you want full
airflow, this is your only option, and if you're remotely mechanically able it's not hard, but i still had to let you know in advance.)

After doing this mod, you will need to adjust the secondary metering to suit. After doing this, my old CK rods/G-hanger set-up was woefully lean. Stepping up to CC rods has put me right back in order.

Other Tuning
The stuff mentioned herein is more advanced and requires you to get into your carb. At this point you definitely should have a manual for your
carb, and should have a good understanding of how it works.

Primary Enrichment
If you have a part-throttle flat spot, or feel you just need a little more enrichment, look here. This requires you to remove your airhorn and take out the plug covering the rich stop screw. By manipulating the travel of the MCS, you can make your car run richer/leaner to suit. As longs as you don't go too far, it will still keep low emissions and fuel economy. But under WOT/enrichment mode you would have the extra fuel. Give it more travel and it gets richer, less travel is leaner. Don't go under 3/32 travel though, that would be too lean and probably mess up overall driveability.

Adjust this in 1/4 turn increments. I bought a kit with the proper tool to turn it, and a travel indicator/idle air bleed tool for $4 at Pep Boys (ask for a mixture adjusting kit, listed with the rebuild kits for the carb.)

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