Secondaries aren't opening on my 1976 carb I rebuilt...

Most "hot air" choke set-ups have a fresh air supply to the metal tube going to the intake crossover passage coming out of the rear of the airhorn. This insures that ALL of the air to the choke and on into the engine is filtered.

You can destroy the rings in an engine pretty quickly sucking unfiltered air into it. It's even worse if you travel gravel roads or live in dry dusty areas.

Secondary choke pull-offs seem to be a big part of this discussion even though they have nothing to do with any of the issues going on. The read Larry put up is accurate but only for some of the pull-offs that were used. Many of them did NOT use a secondary spring to work against the thermostatic spring and no matter how small the restriction in the supply or big the vent hole they can't be slowed down enough to allow the choke flap to be opened further than the primary pull-off. In other words in really cold weather on a cold start adjusting one to open the flap further than the primary pull-off results in a lean condition and engine stalling after 3-5 seconds of start-up.

Rochester knew this and moved to one pull-off for most Q-jets after 1979 and with few exceptions they also got away from hot-air chokes and went to electric. I like and use electric chokes here with a 100 percent success rate. They just work better because they are not relying on engine exhaust gasses as a heat source. That system works fine but there are several leak points and the metal tubes rot out. Not to mention that more times than not folks move to an aftermarket intake that will not even have the provisions for the factory heat tubes in the first place, blocking or restricting exhaust crossovers, installing spacers, etc, etc. The folks making those intakes certainly did this to expand their market since the end user now had to buy an aftermarket carburetor with an e-choke to have a working choke in the first place. Some more educated users converted their factory carbs to e-choke but most folks buying those parts also bought into the BS that removing an 850cfm 1971-74 Buick 455 carb (for example) and replacing it with an outdated/obsolete design Edelbrock 600cfm electric choke AFB clone was going to IMPROVE performance someplace......NOT happening. I'd also mention here that the AFB design was obsoleted clear back in 1968-69 as it was unable to get most engines thru ever tightening emission standards introduced at that time. Even so the effective "marketing" by the companies making them has made those pieces of bovine excrement a main-stay in this hobby and they are still selling zillions of them to this day....FWIW...........Cliff