Air valve connection on rear of TQ carbs

[GTO Geoff]

I asked this question on another forum, got lots of 'answers', but none that answered the specific questions I asked.

I have two 850 Comp Series TQs, one with screw in jets, one with push in jets. The fitting at the rear of the carb where the air valve diaphragm [ AVD ] plugs into is a brass tube with a small hole, about 0.015", in it to act as a restriction.

I have only seen these brass tubes on two other TQ carbs, both 9000 series. One was used on a Aussie Ford. Have never seen a 6000 series TQ with the brass tube, only a steel tube for the hose to fit over, & the tube has NO restriction in it.

Going by part #s, the CS TQs were the very first TQs that were produced.

My questions concern the AVD connection on the rear of the carb.
[1] The brass tube version severely restricts the opening rate of the AV. Seems very odd that a performance carb that is going to flow a lot of air has this restriction to air flow, but production carbs do not?
[2] Assuming that the CSTQ was the first TQs to be made, is it possible that Carter thought the restrictive brass tube was too restrictive, & switched to a non-restrictive steel tube on subsequent models [ relying just on the small hole in the AVD for restriction ] ?
[3] Has anybody drilled out the brass tube & if so what difference did it make?

Thanks.

[rgalajda]

That is a choke pull-off and (air valve diaphragm ) to control the secondary air valve .
The restriction controls the bleed off allowing the air valve to open at a rate that will provide smooth secondary operation
Quadrajet carbs use a restrictor in the diaphragm tube to control early opening of the secondary air valve . Removing it will create a bog. It usually is 1.5 to 3 seconds release time depending on application. It also serves as a choke pull-off.

This is how it is explained in the 1972 Thermoquad manual.

PASSAGE IS OPEN TO MANIFOLD
Vacuum is routed to the choke diaphragm
through a passage which connects to a chamber
in the bottom of the throttle body. This
chamber is open to manifold vacuum regardless
of throttle position.
VACUUM BLEED PASSAGE ALSO USED
In addition, a bleed passage extends from the
diaphragm vacuum passage to a small port in
the secondary bore on the choke diaphragm
DIAPHRAGM VACUUM DROPS SLOWLY
When the secondary throttle valves open,
manifold vacuum drops abruptly but the drop in
the diaphragm is slowed by restrictions at the
bleed port and in the vacuum passage to the diaphragm
DASHPOT ACTION CONTROLS VALVE
As the vacuum bleeds off, the choke diaphragm
acts as a dashpot, preventing the secondary air
valve from opening too quickly. As a result,
incoming air flow opens the air valve at a rate
which provides smooth pickup of the secondary
discharge nozzle operation.
VALVES ARE LINKED TO DIAPHRAGM
The bowl cover also houses a conventional
choke valve and a spring-loaded secondary air
valve. However, unlike similar carburetors, both
valves in the Thermo-Quad carburetor are
linked to the choke vacuum-kick diaphragm.
CHOKE DIAPHRAGM DOES DUAL DUTY
In this case, the choke diaphragm does two
jobs. Besides its regular vacuum-kick function,
the diaphragm also retains the air valve to keep
it from opening too quickly. If the air valve
opens before adequate flow begins at the
secondary nozzles, flat spots can result.

[GTO Geoff]

Rg.
Thanks for the reply. I know what that restriction/tube does. That was not my question. Every TQ AVD I have seen has had a restriction [ small hole ] in the end of the nipple where the rubber hose attaches, to control the release rate & this includes the CS TQ. For some reason, the examples I mentioned use a brass tube with a 0.015" orifice in the carb body for the AVD hose attachment. This orifice is MUCH smaller than the hole in the AVD nipple, & so would control the AVD release [ make it slower ].

Further, the AVD used on Chrys 6000 series was the SAME as used on the CS TQ [ going on memory, I believed it was stamped 447 ]. It had a faster release rate than others I have seen.

Will see if I can get some pics.

[rgalajda]

The one I have here is a 9127s / 1977 440.
The choke pull-off or AVD has a .033" in the nipple "C" and releases in 1 sec. No number on it.
The "B" port in the secondary throttle valve measures about .040"
There is no restrictor at "A" in diagram and is not open to intake manifold vacuum ( as in picture )

I think they used different configurations over the years because this carb is not the same as the picture below from the 1972? manual

I have measured some quadrajet pull-offs (AVD) with .015' restrictors and they release in about 2.5 seconds

Can you remove the brass tube to replace it with another tube to do some experiments?

[rgalajda]

Forgot the photo

[GTO Geoff]

Here is a pic. CS TQ on top. Base of a #6318 production carb underneath.