[SuperHart]

I will throw in my two cents worth here. I agree with the statement about using vacuum secondary carbs for a dual quad setup. First, you rarely if ever see any factory dual carb setup using mechanical secondary carbs and for probable good reason. Most of the cars that used dual carbs were relatively heavy vehicles and didn't rev up all that fast. The vacuum pots were much more forgiving in that respect. Secondly, the vacuum secondaries only open as fast as the engine needs the extra CFMs. If you look around you will see that it is mostly the drag racers using mechanical secondary carbs. The majority of our engines do not displace a gazillion cubic inches and even a total of 780 cfm on a whoopass FE is plenty.

Setting up a pair of vacuum secondary carbs isn't really that complicated. First, each carb should be initially run on a single carb motor and adjusted for best performance. That way, when the carbs are installed together on the dual setup motor you stand a better chance of being "in the ballpark" from the git go. From experience I can tell you that the carbs in a dual setup must be adjusted to a finer level than single carbs because any misadjustment will be magnified (1 + 1 = 3) in a dual carb arrangement. One of the main things to do is be sure that the butterflies of each carb are properly adjusted to provide full closure in the throttle bores. Any misadjustment here will result in the inability to properly adjust idle speed. Also, be sure to purchase Holley kit #20-73 which is a pair of quickchange vacuum pot covers each having a small hose nipple. You want to connect these two vacuum pots with a length of vacuum hose to equalize the vacuum secondaries. These covers also allow quick and easy vacuum secondary spring changes.

IMHO, the best Holley carbs to use for a good running dual quad setup are the #8007s which are rated at 390 cfm. I run a pair of them on my 351W with S&H air cleaners and get not only good gas mileage but blistering acceleration. I ended up using them just the way they came from the factory, no jet or spring changes. In addition, I no longer smell like exhaust fumes after a drive like I used to with a single 650 DP Holley carb. I also know a guy with a 427FE who installed the same carbs and he loves them, saying his motor runs stronger with the 390s than with the original 600s. Overcarburetion is a real detriment to good performance. Don't even try to adjust the idle mix screws the same as on a single carb setup. Your one engine will now be drawing idle mixture from two carbs instead of one so each carb will supply roughly half the requirement. This means that the idle mix screws will most likely be turned inward more than if a single carb was used. Once you get everything adjusted you will probably never want to go back to a single carb setup. The duals just look so right in a Cobra, surpassed only by a set of Webers or Wayne's Stacks.