After the epoxy cured, Sean flipped over the main body and went to work on the idle tubes. These tubes meter the amount of fuel available to the engine at idle speedsessentially, they function as the jets of the idle circuit. Proper sizing of these tubes is a function of the camshaft profile and intake design. Sean drills the tubes anywhere from 0.031-0.049 inches. Since our camshaft was only slightly more aggressive than the stock 401 bumpstick, he selected a 0.039-inch drill bit to enlarge the tubes 0.004-inch over stock for a bit more fuel at idle speeds. Sean pointed out that its absolutely necessary to know the customers engine specs when making these kinds of adjustments to Quadrajets. These carbs are very sensitive to engine vacuum, so even a 0.001-inch variation could sour your driveability.