Originally posted by 64fleetside:
I think air flaps ARE vacuum secondaries, usually the diff is double pumper vs. vacuum secondary. A vacuum secondary carb engages the secondary slowly(to prevent bogging) whereas the mech sec is instant(mech linkage) & uses a second accelerator pump to prevent bogging. The air flaps on a Qjet is just a diff way of doing things than the vacuum "pot" on a vac sec Holley. Usually vac secs are better on the street & double pumpers more suited for racing or hi-perf. Brian
The flaps are generally called "secondary air doors." They're separate from the secondary butterflies and are there to prevent air flow thru the secondaries until "allowed". For example, when the choke is engaged, the secondaries are usually locked out so that the choke function isn't bypassed (and it keeps hot shoes from completely opening up a cold motor.) This may or may not be done by the air door depending on the type of carb.
A secondary air valve is used on many Q-jets to delay the opening of the air door so that the carb doesn't open up all at once. Removing the air valve linkage can be an interesting experience. I did this once on a 406 V8 with a Q-jet and didn't tell my wife I'd done it. She stood on the gas and it scared the hell out of her because it opened up "right now!" and broke the rear end loose big time.
Another feature of the secondary air door is to limit the amount of air that passes thru. This way the carbs become a "variable cfm" device, only supplying as much air as the motor can pull. This makes it easier to use large capacity carburetors on small motors. For example, the electronic Q-jets used in OBD-I motors (81-88 or so) could flow upwards of 700 cfm, but were used on 305 motors that normally couldn't handle anything near that much carb. But when running at WOT, the secondary air door would only open partially, restricting the effective cfm rating to something the 305 could handle.
Otoh, mechanical secondary carburetors (and not just double pumpers) open the secondaries based on the mechanical linkage without any other restrictions. So at WOT, both sets of butterflies are completely open. This makes matching the carb's capacity to the motor a little more critical to get optimal performance without "overloading" the motor's capacity.
Just my take on it.
