Carb tuning is a familiar procedure to old racers, something practiced from an early age, when anyone with a good ear and a screwdriver would put a tune to their wheels on a Saturday afternoon. These days, with OEM fuel injection systems having been the norm for a couple of decades, we've run across guys who have never even driven a carb-equipped car. Without good old learned experience, the art of carb tuning is filled with mystery. Really, there is quite a bit of flexibility in the metering design of a carb, and with a little tweaking in the right places, carb function can be tailored for the task at hand. We installed an Edelbrock four-barrel atop the tired 318 in our unrestored and ratty '68 Barracuda, and it worked well without any fiddling. Not content to leave well enough alone, we broke out our tuning kit and screwdrivers to fine-tune the carb for the application at hand. When we were done, we didn't find a miraculous boost in horsepower from our stock 318, but we definitely improved the efficiency and fuel economy with the razor-sharp tune. Let's look at the inner workings of an AFB-type carb and review the tuning parts and their functions.

Float Your Boat
There's no use trying to tune a carb if the basics are way out of whack. First, ensure that the floats are properly set. Carb floats are attached to the needle and seat valve and regulate the fuel level in the bowl. Floats set too high will cause the carb to run excessively rich, while having the floats too low will lean the mixture and potentially cause the carb to run out of fuel at wide-open throttle. The float adjustment on Edelbrock carbs is an easy two-step procedure. The air horn (or top of the carb) needs to be removed since the float adjustment is internal. Open the carb, and with the air horn inverted, gauge the distance between the top of the float and the air-horn gasket. Edelbrock gives a spec of 7/16-inch for float height. No special tools are required here; just use a 7/16-inch drill bit as a gauge. Bending the float arm adjusts the float height. Support the pivot end of the float as the adjustment is made to avoid pressure on the needle valve while bending the arm.

The second step is to adjust the float drop. The float has a tang on the pivot end that bears upon the seat to provide a positive stop. Measure the float drop with the air horn upright, and adjust the float drop by bending the stop tang. Float adjustments take only minutes and should be verified before any serious tuning or carb changes are made.

Idle Time
Setting up the idle is the most basic level of carb tuning. For the newbies, here's the simple procedure: Bench-set the idle mixture screws by screwing them in until they bottom, and then back each out approximately three turns. AFB idle-mixture needles work conventionally-out for rich, in for lean. With the engine running and warm, adjust the idle speed to the desired rpm, and then turn your attention to the mixture screws. The idea is to set the screws a bit rich, off (out from) lean-drop. With an open-plenum manifold, turn both screws in evenly a half turn at a time until the idle rpm drops noticeably. This is the lean-drop point. Then, back each screw out about 1/4-1/2 turn until the idle quality recovers, and it's done. With a divided-plenum manifold, the same technique can be used, but a final fine-tune can be made individually on each mixture screw using a similar technique.

If there is a significant upward rpm change as the idle mixture is zeroed-in, readjust the idle speed screw and double check the mixture screws to be just out from lean-drop. This technique uses rpm as a gauge of idle quality, which we gauge by ear, although some prefer to find lean-drop with a vacuum gauge or tach.