Hi all,
While this is slightly off topic it IS at least in reference to a teener, specifically my v-8 converted teener.
It's a 350 sb and it uses a 1970 Qaudrajet 4MV carb. I know alot of folks don't like these but when they're dialed in right they give a good blend of economy and performance.

My question is twofold-
1. No doubt you're familiar with the secondaries "air flaps" that are visible on the top of the carb but down deep are the secondary "baffles". You can manually open them up all the way wide but on the car the linkage only open them a crack. I imagine further opening is a function of engine air flow and demand. Or am I totally wrong and they are in fact supposed to open wide via linkage?
2. Has anyone ever experienced constant vacum at the port feeding the secondary dash pot? It's the one that's connected to the secondary air flaps on top. It's supposed to stop it's vacum when you get out of the primaries and get into the secondaries thus allowing the air flaps (the ones on top that you can see) to open. For some reason mine has constant vacum and as a result I can't get my secondary air flaps to open (so I've had to disable that dashpot and cap the vacum port).
That dashpot is supposed to prevent the air flaps from "bouncing open" and I assume I'm gonna get a bog when I do quick shifts if I don't resolve this problem :-(

Your experience is appreciated.
David

The air flaps on the top of the secondaries move the metering rods up and down based on airflow. The butterflys on the bottom are controlled by the throttle linkage. there is no vacuum or mechanical connection to the air flaps on top.

Some of the quadrajets have a lockout tab that keeps the secondaries from opening until the choke is fully opened. This is supposed to keep you from romping on the gas when the car is cold.

The jets and metering rods are much harder to replace than the Holly or Edlebrock carbs out there. GM tuned the Q-Jet on a dyno for a particular engine size/cam combo. So a 350 truck carb won't work correctly in an 350 Vette of the same year.
The metering rods and jets are not easy to get either.

I would suggest an Edelbrock EFI conversion. If you insist on keeping it carbed, then change the Q-Jet out for an Edelbrock street carb.

I am planning on making a change but I want to fix this problem as I have other things I need to spend money on before that.
The baffle secondary stop had been removed ages ago.
I can't help but wonder what is preventing the lower baffles from fully opening....
I tried adjusting the rod a little and while that helped it didn't get me all the love. Mebbe the spring needs some kind of adjustment?

when the throttle is all the way open the secondary throttle plates [the lower plates] do not open all the way?

are the primary [small diameter] open all the way

this might help https://www.youtube.com/watch?v=WfuKmz2_m7k and https://www.youtube.com/watch?v=ZO6JBEdb7F4

Correct! The primaries open all the way but the secondary baffles do NOT open all the way and the secondary baffle choke/stop preventer has been removed.
TY
David

I've looked at that guys entire series and have yet to see this problem addressed.

This was MANY years ago but I had a Carter Thermo-Quad on my Olds 442 and I seem to remember that you could adjust the top secondary plate opening with a screw that tightened a spring and there was a lock screw that held it in place. I think they were mainly vacuum controlled though.
BICBW

How have you verified this problem?

Vacuum is not the only control on the secondaries. Air velocity through the carb opens them as well. Vacuum closes them, air velocity opens them.

This cannot be checked in the driveway. It can only be checked on the road or rolling dyno. The system needs load and speed, otherwise they will stay shut.

The Quadrajet was designed for multiple displacement engines. If you don't have the pumping volume (air flow) it will not open all the way. This is the design. So in some cases (small block engines) it will never open all the way. If it does open all the way on a small engine, the performance will suffer.

"baffels"?

there are secondary throttle shaft and plates, and there are secondary air valve, if you are talking about the flow directors that are between the two they do not move.

the air valve will not open all the way when the engine is running until the vacuum signal drops on the primary side and the vacuum servo allows it to move. also the secondarys will not open while the choke is in operation.

on second thought....

have you checked to see if there is interference from a gasket or the intake manifold that is causing this?

either the linkage or the throttle plates?

Your referring to the air flaps on the top of the carb. That's not where the (main)problem is. I'm referring to the plates internal and at the bottom of the carb. The diagram I have refers to them as "baffles"
I am having a problem with the air flaps but due to the vacum supply for the dash pot that controls the air flaps from "bouncing". I have temporarily dfeated this problem by unhooking the vaccum line and capping the port.
At this point, I'm trying more to solve the issue of the lower baffles not opening all the way when the linkage is open wide.

I believe you are referring to the "air flaps" on top of the carb. That is NOT what I was referring to. I am referring to the lower flaps (or as the diagram I have refers to them as "baffles"). They are not opening fully when the throttle is wide open.

As previously stated:
"Baffles" (and that's what the diagram refers to them as) whch is to say the lower flaps in the base of the carb have had the choke/preventer pin removed (ages ago) so that's not holding them back.
The servo you are referring to for the upper "air flaps" is holding them closed (when hooked up) no matter how long you allow for warm up nor within any throttle position after warm up (which means the port from which a vacum source is powering that servo is not un vaccuming in any range or basically locked when attached to the correct vacum line) I have disabled it and the servo unloads allowing the air flaps to open. But that said, a bog is surely to occur as the air flaps will bounce during shifts.
Click to view attachment Scondary air "baffles"
Click to view attachment Secondary "air flaps"

Here is the throttle linkage that controls the primaries and the rod controls the secondary baffles (the flaps located down inside the carb).
When the linkage is fully opened it fails to wide open the secondary air baffles. You can manually open them by hand wide open. And no, the gasket is'nt preventing them as this was my first thought so I tested with the carb off the engine. Same results. Messing with the rod will only achieve premature opening of the secondaries before the primaries have fully opened (creating a bog)
Click to view attachment

pm sent

And here is the dashpot that does not release (when hooked up) thus freezing the secondary upper air flaps (different than the lower secondary plates called "baffles")
Suffice to say the zip tie from the PO was a lame attempt at addressing a malfunctioning choke. That is a seperate issue that I have rectified by installing a simple (and stock) divorced choke system and had nothing to do with the secondary upper air flaps.
Click to view attachment

You are using a spread bore QJ manifold, right? The secondary baffles plates have always opened 100 % at full throttle on my QJ equipped 350 Chev.
Some tips that I've learned with this carb on my almost stock 3,300 lb. '69 Camaro...which finally broke into the mid 13s with the quadrajet :
The small primaries give you excellent low end response. The primary section has removable jets and metering rods, sourcing those today might be a local carb shop. The secondaries have no rear removable jets, but instead two cast-in -place bores in which the metering rods are suspended. The metering rods are removable (for narrower or wider dia. contours =mixture) , and the metering rod hangers (removable) are also made at different heights (stamped with various letters). The best spring setting on the secondary baffles is just a slight push on the baffles where they want to return without too much spring tension. A common defect on the QJ is the fuel chamber...they leak. A carb shop should have the fix-it freeze plug kit for replacing the oe leaky plugs. When these leak it makes the car surge, idle weird and sometimes undriveable. I also fine polished the big secondary venturis for better airflow. With Chevy cam #3863151, some port matching, open headers (colder spark plugs + bigger primary jets) and a 3 speed automatic, it ran bracket at consistent 13.5 @ 105 mph
Compared to the Holley or Carter 4bbl carbs, where I would be running high 13s to low 14s at 97-99 mph.
Hope some of this helps !
Here's a pic from Lions Dragstrip 1969,
MartyClick to view attachment

This was the problem. The spring that creates tension on the linkage that opens the lower secondaries did not have enough tension when who ever last (before me) had the carb apart. Fixed the problem. Lower secondaries now open almost 90 degrees.
Click to view attachment

Thermo quad was basically like a QJ with the spread bore style . The innovation was in the center section (which holds the fuel bowl chamber), it was made from black phenolic resin. Principal was to isolate heat from the intake manifold , creeping upwards towards to the carb...causing vapor lock. I simply ran a cool - can stuffed with ice and the Quadrajet ran all sessions without a hiccup. Phenolic resin spacer blocks are used as intake insulator spacers on 930 hot rods, and all sorts of other forced induction engines.
Thermo quads were factory equipment on many Mopar Darts, Dusters+ Roadrunners in the day.
Half Moon...good diagnosing !
Marty

now get those plugs epoxied up!

I bet it is the spring that creates tension on the linkage.....Oh, never mind

Um..., very helpful Dr