Carb problem?

[mike561]

I was cleaning out my carburator today and when i gave it the throttle i noticed the 4 barrel isnt opening (the 2 big flaps on the top of the carb) only the little bottom one would be open. i can open them by hand but when i give it the gas they dont open by themselves. i studied and it seems theres some type of vaccum thing that may open them. any ideas of whats going on???? the car starts and runs fine, but i notice a big power loss when im driving and stepping on the gas. i just had a big tune up as well. its the 2 big flaps on the top left side in this pic:

[silicon212]

If you have a power 'loss' as in it bogs down, then it is likely they are opening too fast. If they don't open at all, then you just won't experience full power. The secondary butterflies are what is called an air valve, and it is operated directly by engine vacuum. There is an adjustment on it, a screw that is locked in place by a setscrew.

[PeteA216]

While the engine's off they will not open on their own, and while the engine's running I wouldn't keep the throttle that high to see the secondaries open long or you could blow something. I had a similar problem with my Q-Jet, when I'd stomp on it i'd get an immediate bog down, and then the engine would pick back up after about a full 1 second of bogging. I changed out the Q-Jet with an Edelbrock performer (not because of this, but b/c of my computerless application) and the problem was gone for a few months then came back, but was less severe. I never looked into it though because the car went off the road shortly after for an engine swap. It's in progress right now.

[mike561]

Yeah i moved the throttle while the engine was on, but the 2 big flaps diddnt open or move at all, and it doesnt bog down, but just takes a while to get up and going. so there are adjustment screws for the flaps to open though for the vaccum? where would this screw(s) be? can anyone provide a pic? i was thinking of getting a performance carb down the road anyway, but i just want to get the best out of the stock one as i can for now since im broke at the moment.

[silicon212]

The stock carb is actually your best bet. These will flow up to 780 SCFM depending on engine vacuum - in other words, with the E4ME Quadrajet, you can have up to 780 SCFM without overcarbureting the engine. You also get to retain your cars' computer system which provides the best possible performance for any given driving condition. You can't control the spark curve on your mechanical advance distributor the way the computer can, I don't care how good of a tuner you are. The computer runs the timing as agressively as possible without encountering detonation (spark knock) for the best possible performance.

[mike561]

sounds like i should stick with the stock then, sounds expensive to get another type. i still want to try to figure out how i can get the flaps to open though. it sounds like its just a matter of an adjustment, if only i could figure out where these screws are though.

[mike561]

sounds like i should stick with the stock then, seems expensive to get another type also. i still want to try to figure out how i can get the flaps to open though. it sounds like its just a matter of an adjustment, if only i could figure out where these screws are though.

[Blue Bowtie]

There is a secondary air valve (the flaps) spring which can be adjusted.

From our friend, Ed Maher:

Tuning for Performance with your Computer-Controlled Quadrajet
Here for your digestion are some easy mods, with brief explanations, that you can make to any computer controlled Quadrajet. They require only simple tools, and most are free. If you have any questions, or need some advice, email me. -Ed Maher

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 *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 rarer.

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 780 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.

Thanks, Ed.


And just a few random bits of information:


SECONDARY METERING RODS

Tip Length Legend:

• LL: Extra long power tips supply richest mixture starting at 60º of air valve opening.
  Never use in performance applications using greater than 70º air valve opening.
• L: Power tip starts at 70º air valve opening.
  Tip considered to begin at that part of the rod that is within .003" of rod's minimum diameter
• M: Power tip starts at 80º air valve opening.
  Tip considered to begin at that part of the rod that is within .003" of rod's minimum diameter
• M/S: Power tip starts at 80º air valve opening.
  Tip considered to begin at that part of the rod that is within .005" of rod's minimum diameter.
• S: Power tip starts at 90º air valve opening.

[mike561]

So its just a matter of an adjustment rather then a problem with the carb then?

[Blue Bowtie]

Unless the secondary AV is stuck, and if you can easily open it by hand, the adjustment of the spring and vacuum actuator linkage should get it working.

[mike561]

Thats probably what it is, because i can definetly open it easily by hand, or by finger i should say.

[mike561]

Ok well i just took the car to the shop and had them adjust it no charge. it gets up and goes MUUUCCCCHHH better now. i think the timing needed adjusting too. thanks everyone though for all your input.