Quadrajet Experts Please Help!

[DANTIP]

My 72 has a 4MV Quadrajet carb. The engine starts and runs great (seemingly perfect choke operation) but the secondaries do not seem to ever open. I somewhat understand that the divorced choke prevents the secondaries from opening up but as it warms up and relaxes, the secondaries are able to open. I drove the truck for about 15 minutes and brought it back into the garage and looked at the linkage on the passenger side. It looks like the vacuum diaphram is pulled in and preventing the secondaries from opening. Please help me!

[Sport/Truck]

Try unhooking the vacuum line going to the vacuum port. (cap it off) and see if it allows it to release. Can you manually pull it out to allow the secondaries to open, does it feel stuck? I checked mine and it's released (cold) and if I pull it in it does prevent the secondaries from opening.
s/t

[DANTIP]

I was able to manually overcome the force of the vacuum diaphragm but when I released the force, the diaphragm pulled it back to where the secondaries are unable to open.

[Sport/Truck]

But with the engine off it's fine? (eliminating a bad diaphragm)

[JimKshortstep4x4]

There is a lockout of the secondaries on the right hand side of the secondary throttle shaft. If the choke is not completely open it will keep the secondaries from opening. Usually the vacuum pot will not stop the secondaries from opening as there is little manifold vacuum when you have the throttle wide open with the engine winding up.

Jim

[Jim_PA]

If the engine is running in neutral, the diaphragm will be pulled shut, and not allow the secondaries to open... If you disconnect the vacuum hose while the engine is running, you should see the diaphragm relase, and allow the air flap to move freely when needed.

[DANTIP]

I found an Edelbrock pdf that describes choke rod length adjustment. Does this procedure apply? If not, how do you changeout or adjust the divorced choke rod and thermal spring?

Thanks!

[Blazer1970]

The secondary air flapper valve on a Q-Jet will never open when you are just revving the engine in the driveway. The vacuum dash pot holds it closed unless the engine is under a load (low vacuum condition). I think a lot of people expect to see it open when they just rev up the engine in neutral, and if the carb is working correctly, that just isn't going to happen.

[68C15]

that secondary pull-off is designed to do 3 things.
1; it opens the choke part way right after a cold start
2; it regulates the speed at which the secondaries open under load. as vacuum decreases it gradually allows them to open. too fast and the engine sputters due to an aver rich condition.
3; it pulls them shut when you back off the loud pedal

[DANTIP]

The biggest reason I suspect the secondaries are not opening is if you floor a Quadrajet-powered V-8/AT, your AT will typically downshift, your secondaries open and the performance/acceleration is excellent. My TH-400 down-shifts but my engine performance doesn't seem to improve much. My Q-Jet seems to be in excellent condition, the choke, high-idle, warm-idle seem to work normally and all seems normal except for the lousy WOT performance. If the 402 was just tired, I would think it would smoke, or use oil excessively, but that's not the case. Secondaries failing to open would explain everything. Is there any way to determine if the secondaries are in fact opening? Am I missing something?

[Wrenchbender Ret]

Lets do this systematically. First see if the throttle valves in the bottom of the carb are open ing. The choke must be fully open as mentioned earlier. Have someone push down on the throttle all the way.(engine off). Watch to see if the butterflies open. Sometimes the cable gets stretched or the carpet wont let it go down far enough. You can put a block of wood between the rod where the cable hooks to & the firewall & bend the rod the pedal hooks to up a little to get more travel. After you you get that part rightT with the engine running put a little pressure on the air valves on the upper part of the secondary & remove the vacuum hose from the vacuum chamber. The butterflies should open after the rod comes back. You can run without the rod to the butterflies but you will probably have to increase the spring tension on them.

[kwmech]

Originally Posted by Jim_PA

If the engine is running in neutral, the diaphragm will be pulled shut, and not allow the secondaries to open... If you disconnect the vacuum hose while the engine is running, you should see the diaphragm relase, and allow the air flap to move freely when needed.

Partially correct..........there is also a spring tension adjustment on the side for the flaps. From the bottom there is a small allen screw and going straight in there is a small flat blade screw. If you loosen the set screw, you can then adjust the amount of tension on the return spring. Not enough tension though will allow the flaps to open too quickly and create a hell of a lean condition when attempting to go full throttle

[DANTIP]

Secondaries were not locked-out, the cable was pulling far enough to fully open the lower blades but the secondaries were not opening. Eureka!!, The spring tension on the air valve was set too high (1 full turn). The secondaries were never opening. I found a Quadrajet 4MV tuning guide on a Buick web-site that says to start at 1/4 turn of spring tension and drive the vehicle. If it boggs under WOT, increase tension setting 1/8 turn. Keep repeating until there is no bog/hesitation. 1/2 turn seemed pretty close. It felt like I was being pushed by another vehicle from behind when the secondaries opened under WOT. Amazing Difference!

[Jim_PA]

Originally Posted by DANTIP

Secondaries were not locked-out, the cable was pulling far enough to fully open the lower blades but the secondaries were not opening. Eureka!!, The spring tension on the air valve was set too high (1 full turn). The secondaries were never opening. I found a Quadrajet 4MV tuning guide on a Buick web-site that says to start at 1/4 turn of spring tension and drive the vehicle. If it boggs under WOT, increase tension setting 1/8 turn. Keep repeating until there is no bog/hesitation. 1/2 turn seemed pretty close. It felt like I was being pushed by another vehicle from behind when the secondaries opened under WOT. Amazing Difference!

Another good way to do it is to disconnect the choke pull-off (plug the line on the carb, of course...) and start with a tight spring. Under different driving conditions, floor it. If no bogs anywhere, loosen the spring 1/8 of a turn.... repeat driving procedure... when you loosen the spring enough to notice a bog under any driving condition, hook up the choke pull-off, and see if the restriction of the pull-off is enough to keep it from bogging. If you connect the pull-off, and the bog disappears, you have it set where it needs to be.

There's lots you can do, including modifying the choke pull off to release faster, but it probably isn't necessary if you aren't trying to totally maximize performance.

[69GMCLonghorn]

Last time I had a problem with a Quadra-puke, I solved the problem without ever opening it up: I INSTALLED A HOLLEY!! problem solved... (that should start some good arguements! HAHA)

[UltraBlue71]

69 HORN--
I'll take all the old Quads you have, you can keep that leaky, gas hog HOLLEY, it aint 1976 and gas aint .65 cents a gallon!

[Jim_PA]

Originally Posted by 69GMCLonghorn

Last time I had a problem with a Quadra-puke, I solved the problem without ever opening it up: I INSTALLED A HOLLEY!! problem solved... (that should start some good arguements! HAHA)

Come back when you have 100,000 + miles on it, and haven't touched a thing.

[69GMCLonghorn]

Originally Posted by Jim_PA

Come back when you have 100,000 + miles on it, and haven't touched a thing.

Been there, done that, bought the T-shirt... twice! I had a '71 Chevelle that I sold after putting about 110,000 on it... with a Holley, no rebuilds. Also had a '69 C/20 that had over 200,000 on it, about 120,000 of which was on a Holley... It now has well over 300,000 on it, and I believe the current owner hasn't done much to it, but not sure.

I LOVE THIS! We could go back and forth all DAY and never prove a dang thing, one way or another!! Great stuff!

[Jim_PA]

Originally Posted by 69GMCLonghorn

Been there, done that, bought the T-shirt... twice! I had a '71 Chevelle that I sold after putting about 110,000 on it... with a Holley, no rebuilds. Also had a '69 C/20 that had over 200,000 on it, about 120,000 of which was on a Holley... It now has well over 300,000 on it, and I believe the current owner hasn't done much to it, but not sure.

I LOVE THIS! We could go back and forth all DAY and never prove a dang thing, one way or another!! Great stuff!

Holley schmolly, how good is your gas mileage, and we need 1/4 mile times. Holley = over priced snake-oil gimmick garbage.

HEY, LOOK AT ME, I GOT A HOLLEY STICKER!!!

Replacing your qjet with a Holley is like being a ricer with big fart can mufflers and wings. They holley isn't going to make your car any faster. Actually it's worse, becasue the fart can muffler won't hurt your gas mileage.

[DANTIP]

The best carburetors are the ones that YOU can get to work. That is why some people like Q-Jets, Holley, Edelbrock, etc. I have had good luck with Q-Jets and Edelbrock's (especially #1406). My luck hasn't been as good with Holleys.

[SCOTI]

Originally Posted by DANTIP

The best carburetors are the ones that YOU can get to work. That is why some people like Q-Jets, Holley, Edelbrock, etc.

Agreed.

Neither brand should be dismissed (gas hog holley; quadra-puke??). Carbs that aren't tuned properly or in poor condition will have symptoms of poor quality (which has nothing to do w/the brand).

[UltraBlue71]

Look the the primaries on a Q-Jet, then the secondaries, small/huge right

Look at the the same thing on a Holley all 4 are the same size, right?
But bigger than a Q-Jets primaries

Or how about a Predator-
One huge Barrell

Then in theory...
If you keep your foot out of a Q-Jet it should get better mileage.

Granted if you dont have any of them tuned/set up right they wont work worth a hoot.

I have had good experiences with 1406's but Holley products have always gave me hell. But I never had a brand new one either and they seem easier to rebuild than any of them. To each his own I suppose....

[lowbuck72]

how bout some pics of what you adjusted.... such as this spring you changed tension on... i have a blazer with similar problem runs perfect but no four barrel... and i know somebody will say put a holley on it lol i have better luck with the quad in offroad situations i save the holley for the street truck.... but anyway how bout a pic

[SCOTI]

Originally Posted by UltraBlue71

Look the the primaries on a Q-Jet, then the secondaries, small/huge right

Look at the the same thing on a Holley all 4 are the same size, right?
But bigger than a Q-Jets primaries

Holleys standard is the square bore carb but they also make a 650cfm spread-bore (like the Q-jet) carb for L82 applications that works well.