I just put a new (rebuilt) out of the box 7042251 Quadrajet on my 72 U code. Discovered a huge vacuum leak on the rear. Doesn't appear to be threaded and the PCV goes in the front and the brakes to the intake so I don't know what this is. Should I try to press a port in it and cap it? JB Weld? Is this for something rare or a screw up? The large hole in the bottom of the carb.

 

Hot Idle Compensator

 

So apparently something is missing. And I'm guessing this is some rare item? My buddy has 6 carbs sitting on a shelf and not one of them has it. Can it be blocked off?

 

Some models were built w/ them & some w/o the Hot Idle Compensator (parts # 25,26,27,28). I have several Q-Jets - two 7042251, they both have it the other (Chevy Q-Jet) does not. It's thermostatically controlled, when the engine is idling too hot, it opens up to allow extra air to flow into the intake manifold to compensate for extra fuel vapors. Someone forgot to put it back during previous rebuild most likely. With the hole exposed (not plugged) it's a vacuum leak, just plug up the hole, plug up the round hole where seal (part #28) was. I guess you do not have the cover (part #26)?

 

This came with the car and I literally took it out of the plastic and just installed it.

 

You have version B in this diagram. You need the actual hot idle compensator, the cork gasket, and the cover plate with the two screws.

 

I sent you a PM. I’ve got a junk 7042251 with the part you need