Hi Guys, I recently came across a quadrajet carb that had only one vacuum pull-off on it. What's the difference between the two? The pull-off that this one has is the one toward the front of the carburetor plate. There's no provision for the secondary (aft) pull-off, the mounting plate that would hold it is designed to hold only one. The base plate had the vacuum port for the secondary pull-off, but no place to mount/hold it. Clearly, the carburetor number has little value/use in this case. Anybody...anybody....? (It is HARD to do a good Ben Stein impression in just words.) Nathan
All of my 1967 Q-jets for my 430 have the single pull-off. The single pull-off has two functions: to delay/lock-out the secondary air doors, and to properly set the choke flap angle when cold, both during idle (high-vacuum) conditions. The rod pulls the hinge on the secondary air shaft, and has a 90 degree bend about mid-way down which hits a tang on the choke linkage. I believe the second pull off was added to some carbs in the early 70s, and they split the duty: one for the choke and the other for the secondary air doors, with the secondary pull off becoming adjustable with an added spring. Sounds like your carb may be a mutt build from different years. My carbs have the port in the back of the base plate, but that's for the AIR injection system, not a secondary pull-off.
The primary vacuum break on the Q-jet serves a double purpose. Number one, it opens the choke against thermostatic spring pressure, once the engine starts. This leans the mixture just enough so that the engine runs smoothly without loading up. Second, it provides a damping force on the secondary air valves so that they do not tip in too quickly and bog the engine during rapid acceleration. The primary vacuum break works in conjunction with the secondary air valve spring tension. The secondary vacuum break was added on later model carburetors to exert finer control on the choke blade for emissions control. Bottom line is you don't need it, and it has no performance advantage. Don't be concerned with the lack of one on your carburetor.
