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On a mechanically sound car, optimize the ignition timing before playing with the carb. Even if the jets are off, you'll never find the solution until the timing's right.
Typical stock-type distributor curves have too much centrifugal advance built into the distributor. Assuming the use of 92-octane unleaded premium pump gas, less than 9:1-compression small block Chevys like about 10 to 11 degrees total advance in the distributor (20 to 22 as read on the crank), with 16 degrees initial timing at the balancer (for 38 degrees max crankshaft advance).
The centrifugal advance curve should start around 1,200 rpm and be all in by 3,500 rpm. A lightweight car with a big solid-lifter cam (more rpm capability) and deep rear gears will tolerate more overall advance that comes in quicker (as early as 2,800 rpm). High compression ratios call for backing down the timing to avoid street-gas-induced detonation.
Advancing the timing until the car "pings", then backing it off, doesn't always produce the best horsepower. Try advancing and retarding the timing in 2-degree increments to see if the car speeds up or slows down.
Vacuum advance is good for street cars, as it promotes improved part-throttle fuel economy and driveability. Because the vacuum advance doesn't function under wide-open-throttle, you don't lose anything by leaving it hooked up.
Hard-Running street cars may benefit from going to a spark plug that's one step colder than stock (but check for evidence of fouling). With high-output electronic ignitions, gaps of around .040 inch are usually a good compromise between getting a strong spark and preserving coil longevity.
First of all, make sure that you've got the ignition timing set correctly before you start trying to adjust the carb.
Before you make any major carb modification, make sure that the basic adjustments are correct. Set the idle mixture screws to give you the highest vacuum reading or the highest idle rpm using either a vacuum gauge or a tachometer.
Although it won't affect WOT (Wide Open Throttle) horsepower, a proper accelerator pump shot is important to avoid an off-the-line bog, especially with Holley double pumpers. Fortunately, on a Holley you can play with the pump-cam position, shooter size, and pump cams to achieve optimum response. Non-Holleys may have limited accelerator-pump tuning ability, but at a minimum, they always have a rod you can bend. If your car does have an off-the-line bog, a basic rule-of-thumb is: If the car bogs, and the exhaust is black, the mixture is too rich. If the car bogs and the exhaust is "normal" the mixture is too lean.
Jets have a definite affect on WOT power numbers. On a performance application running steep rearend gears where fuel economy is not a factor, jet the primaries and secondaries up or down in equal increments, unless the spark plugs offer a visual indication of uneven fuel distribution. Some carbs use metering rods instead of jets, but the principle is the same.
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