Quadrajunk, secondaries not opening?

Jack Boelte · October 30, 2001

I was trying to get the chole to work on my truck (no luck) and was playing around with the throttle. I noticed that the secondaries don't open up at WOT. I drove the truck and floored and didn't notice a difference in the way it sounded. I'm assuming the secondaries are vacuum operated and not working properly. (No wonder passing on the highway is such an adventure!)

The Haynes manual doesn't really describe the carb operations very well; anybody know what might be malfunctioing? Is there a hose or tube that may need fixin'?

Torque Junkie · October 30, 2001

I believe 76BBSub has a q-jet. He just recently adjusted his secondaires in his in this thread:

Posted: Sep. 24 2001,09:55

finally got around to adjusting the spring tension on my QJet secondary air valve (the rear "flap").

Here's the trick...on the right (passenger) side of the carb, open the secondary air valve by hand. There's a screw on the side that rotates when you open/close the air valve. Right below it is an allen-head socket set screw, it's about 1/8" allen wrench. Put a screwdriver in the one on the side and hold slight tension on it, then loosen the set screw just enough to allow you to rotate the screw on the side. Let off a little on the side screw and count the rotations until the spring tension is released, this is your baseline setting so that if you totally screw up you can go back to the way it originally was. Now, tension the spring about 1/2 turn or so, and don't forget to re-tighten the set screw before you remove the screwdriver from the side screw, or you'll have zero tension. Go drive the truck, if it bogs, adjust it again.

The problem on my Suburban, which I just found, is that the vacuum diaphragm is holding too much tension on the lockout mechanism...I think I will re-drill the orifice on it and see if that helps.

Torque Junkie, hope this helps, let me know if you have questions, comments, etc...

Jeff

I'm sure he'll be in soon and will help you out!

-Mike

bobs409 · October 30, 2001

Was your choke all the way open when you tried this? Some carbs have a way of preventing the secondaries from opening if the choke is on.

I know my old AFB does but that has mechanical secondaries.

Torque Junkie · October 30, 2001

That's a good point too, Bob. My Performer, as well as my old AFB (Essentially the same thing as a performer) have the mechanical lockout that won't allow the secondaries to open while the choke is on even partially.

I would immagine the Q-Jet would too, but I don't know for sure. Logically, however, it would be smart to not have the secondaires open while the choke is set - it would totally defeat the purpose of the choke in that case

Let us know what you find, Jack! Good luck.

-Mike

76BBSub · October 31, 2001

OK, I'm back....business trip to New Orleans Monday and Tuesday, then out of the office this morning until now.

Torque Junkie is right that it may be the spring tension adjustment as described in the previous post.

The QJet does have a lockout for the vacuum secondary butterflies. On the front passenger side of the carb is a small diaphragm. There's a short vacuum hose from the front of the diaphragm to a vacuum port on the carb. There's a slotted "tang" extending from the rear of the diaphragm to which a rod attaches. When vacuum is high, such as at idle or low engine load conditions, the diaphragm pulls the rod, locking out the secondaries. There is a small orifice in the diaphragm, so that when vacuum drops the spring inside the diaphragm forces the rod back out, allowing the secondaries to open. It is concievable that the diaphragm on your carb is not functioning properly. You should be able to test this pretty easily...note the position of the rod with the engine off, start the truck and let it idle, you should see the rod retract in towards the diaphragm. Shut the truck off and immediately observe whether the rod extends again or not. Replacement diaphragms are available from auto parts stores, but the size of the orifice was yet another of the many ways which Rochester fine-tuned the QJet, and I doubt that all the various orifices are still available, or that you could even find out what size is on there now.

Another possibility is that the rod is not extending far enough to unlock the secondaries. You can bend the rod to lengthen or shorten it, this is the way to adjust it.

Another thing to check, with the engine turned OFF, open the secondary air flap by hand and peer into the secondaries of the carb. Have an assistant push the pedal to the floor and make sure that the secondary throttle blades are opening fully. They don't rotate a full 90 degrees, I think it's about 70-75 degrees rotation when fully open. You can check by manually opening turning the throttle, but then you don't know if there is a problem with your throttle cable, linkage, gas pedal, etc, so it's best to use an assistant inside the truck.

Check those things and let us know what you find. I'll continue to think about what else may be ailing your QJet.

Jeff

Jack Boelte · November 1, 2001

Thanks for the replies guys. I'm having problems with the choke, and have been since I owned the truck. Right now I have the choke arm wired (actually zip-tied) open as far as I can get it. So I am pretty sure the choke is all the way open. I will check the vacuum diaphram like 76 BB said.

As for the choke, I installed a new element, the old one read less than 1 ohm. It idled great cold, ran like hammered dog poop warm. I could see that the choke was not closing all the way, but it was mostly open. Also the choke/oil light was on the whole time. I don't have the Electronic Spark Control connected, I've got an older style HEI distributor. I don't think the ESC computer is powered or functioning, none of the instrument warning lights come on. I had to do some custom wiring to get the lock up torque converter working, maybe all this is related?

Thanks again guys!

textruckr · November 16, 2001

Had similar problem after the transmission was rebuilt on the Suburban. I found the electrical connection to the choke oil pressure sending unit had been pushed off when they tilted the engine back. Those models with the Qjet and electric choke have a separate oil press. sending unit - it waits until it sees oil pressure before it send current to the choke heater element and starts a time-delay. This is to prevent the choke from pulling off when the key is in the on position and the engine is not running.

Might also check the choke fuse. I have also had a loose connection at the choke heater element cause that problem too.

Hope that helps.

Texas Truck - r

76BBSub · November 16, 2001

Great reply, TexasTruckr, I am not familiar with the newer versions with electric chokes and such, I hadn't even thought of that. Nice to have some folks around who are familiar with the "newer classics".

Jeff

textruckr · November 18, 2001

With an older vehicle, every day can be an adventure. I tracked that problem down thanks to the wiring diagram in the back of the Haynes manual for that series truck.

I am always chasing some new problem.

Why just today got a strange new vibration at certain speeds. Assuming it is a wheel that needs to be rebalanced.

Oh well. That is why I also have newer vehicles (to run for parts).

Texas truck - r

Torque Junkie · November 18, 2001

Quote from textruckr, posted on Nov. 17 2001,8:23

I am always chasing some new problem.

I know that feeling all to well As soon as I get one thing fixed another springs up. I've spent over $2000 modding and repairing that thing since I bought it back in June, and I've got another $4000 (at least) of stuff planned for it, not including what un-planned stuff springs up (my radiator incident, most recently)

-Mike

76BBSub · November 19, 2001

Yep, between the have-to-dos and the wanna-dos there's always something to do! My '76 is my daily driver, but I have my wife's car available or my Corvette (assuming I'm not all dirty at the time) for parts running. Comes in handy....

Jeff