Ever since I bought my car, I've had the problem that, with the engine fully warmed up, and releasing the gas pedal, the idle speed would, more often than not, not return to normal, but instead stay 100 - 150 RPM elevated.

To make a long story short, the reason was (is...), that the foremost part of the accelerator cable hits, with the accelerator pedal fully released, the AC base, thus preventing the primary throttle plates from closing fully. See red arrow in first picture.
(Which explains, in hindsight, that one of the POs installed a stack of three gaskets between the air horn and the AC base in order to get the AC base a little bit further away from the accelerator cable...)

Now, while debugging this issue, I found something else: With the accelerator pedal fully depressed, the secondary throttle plates would not open fully (which means that, for the past two seasons, I drove around practically on the idle circuit ...). The reason for this also lies with the accelerator cable: WOT is, in my case, not determined by some stop in the carburetor (linkage), but by the maximum travel allowed by the cable itself (see second picture).

Now, in order to remedy these two issues, I could take a shortcut and use a Dremel and other hand tools on the offending parts - but I abhor such pragmatic approaches ... So I'm going to ask you guys first:

Is there anything obviously non original / modified / bubbafied in my setup?

 

Easiest would be to move the cable pivot stud down to the lower hole. This will lower it so it doesn't hit the air cleaner and would also shorten the "throw" distance required to go from closed to fully open plates. If that doesn't produce enough cable "throw" you can always adjust the cable stop rearward in the retaining bracket a bit until WOT leaves no more slack. Since the dust sheath is missing this should be easy to see.

 

Is this your air cleaner?

http://www.zip-corvette.com/ProductDetails/ProductDetails.aspx?pid={d2659509-4d33-447c-a280-16b4cb5295ea}&gid={d3567a2f-cc5a-46e7-a848-953b1d89f75c}&GroupName=Complete+Air+Cle aners&pname=70-72+350+%26+454+w%2fQ-Jet+Air+Cleaner+Assembly&Referer=&Alias= &ptct=SGR-SR&CTitle=&

 

Hi qr,
I agree that it appears the accelerator cable has been modified.
Here's a picture showing the cable on my 71 sb which I believe is the same as used on the bb.
Is that the correct linkage for your bb.... I guess I'm asking is that the correct carb for your engine... or is it possible the cable was modified a bit to fit the carb from another application?
Regards,
Alan

 

No - I have what I believe is called an 'Open Element Style' AC. Like this one. Although I think mine is still the original one.

My carb is 'correct' for my application - and I believe the orignial one. Accelerator cable also seems unmodified, with the exception that the plastic dust shield has broken off.

But the mounting point of the cable on the lever now really seems odd. Even the '70 AIM (should have looked there earlier) shows usage of the lower of the two mounting holes (although the pic is for the base engine...):



So I'll try this approach... Thanks to all!

(But if someone out there has a pic of an original BB w/ QuadraJet - I'd still appreciate it. )

 

I had the same issue with my 70 L-46. Loosen the throttle cable clamp at the rear of the carb, have someone hold the gas pedal to the floor, (make sure you have the choke and secondary air valve open) you then can manually open the throttle linkage while watching the throttle plates, open the throttle until the top/upperside of the secondary throttle plate is pointing at the lower edge of the secondary baffle.

Once you have all these aligned as needed, you tighten the clamp on the throttle cable. The shoulder on the throttle cable may be pulled back off of the clamp/stop.

From Lars quadrajet tuning papers.

"Q-Jet secondary throttle plates should NOT open to the full vertical position or beyond. The secondary throttle plates should open to a position where the angle of the throttle plates points and aligns towards the lower edge of the secondary airflow baffle located in the secondary venturi bore. When the secondary throttle plates are opened beyond this point, turbulence in the secondary side actually decreases airflow. Also, if the secondary throttle plates are adjusted to open over-center, they will, as a result, also open too soon. This will cause a stumble or hesitation going into the secondaries. Adjust and align the opening by bending the primary throttle contact tang."