There are usually two vacuum ports (metal tubes) in the front of carbs. The higher port (tube) is called a timed port as it is affected by butterfly movement. The lower port (tube) is a direct source of vacuum and is NOT affected by butterfly movement.
Most carb suppliers recommend using the timed port, but after reading Woods' (he's a very knowledgeable member here) posts in the "Mileage Quest" thread found in General Discussion, I've changed my thinking and now recommend using the direct port location as the proper source for vacuum advance canisters.
1970, 2WD, F-250, C/S, Dual Tanks, 390 FE (of course), C6, Power Steering, Power Disc Brakes, Dana 60 3.73, Sky View Blue, Ranger XLT
1970, 2WD, F-250, C/S, A/C, Dual Tanks, 390 FE (again, of course), C6, Power Steering, Power Disc Brakes, Dana 60 3.73, Wimbledon White & Sky View Blue, Ranger (almost twin brothers!)
"One of the greatest discoveries a man makes, one of his great surprises, is to find he can do what he was afraid he couldn't do" - Henry Ford