Also, look carefully for any kind of vacuum leak. Driveability problems like yours are often caused by small vacuum leaks that make the engine run lean at part-throttle. If you don't have the vacuum-advance hooked up, please do so. This will help part-throttle response, and the engine will run more crisply. These few changes should solve your problem. Also, make sure to carefully set your idle-mixture screws and make sure they are balanced with the same number of turns out for both adjustment screws. This is a common oversight. Slightly (emphasis on slightly) richer on the idle-mixture screws might also help your off-idle stumble.
If none of those changes solves the problem, you'll have to go into the carburetor. Keep in mind that the longer-duration camshaft reduces the manifold idle vacuum, which reduces the signal to the carburetor. Let's start with the primary metering. Your Edelbrock carburetor is the 750 manual-choke carb with the performance calibration and stock metering rods. The first number stamped on the metering rod refers to the upper portion of the rod (0.071 inch), while the second number refers to the power tip of the metering rod (0.041 inch). The upper portion of the rod sits in the jet, displacing a given amount of area so that when the throttle is at light cruise, very little fuel is contributed to the main metering circuit. As you apply throttle, the power-valve spring (the step-up spring) pushes against the little piston connected to the metering rod. As engine vacuum decreases, the spring pushes up on the piston and lifts the metering rod up out of the primary jet. This moves the metering rod to the smaller power-tip area, which flows more fuel. This is also exactly how the primary circuit works on a Q-jet. When you stretched the little spring, you effectively moved the metering rods up out of the jet sooner, which riches up the part-throttle circuit. This is why the engine liked that little trick.
Our suggestion would be to purchase an Edelbrock recalibration kit (PN 1480, $48.88 from summitracing.com), which includes an assortment of metering rods, jets, and step-up springs. Or you could just order the parts separately. We'd go richer on the primary metering rod to a 0.065x0.037. The stock spring (before you modified it) is rated at 8 inches of vacuum. You will probably need more accelerator-pump squirter. The accelerator-pump squirter kit (PN 1475, $23.88 summitracing.com) comes with pump-nozzle sizes of 0.024, 0.033, and 0.043 inches. Try the 0.033-inch nozzle-that should help. If it doesn't, try the larger 0.043 nozzle. If you want to do a quick fix, you can drill these out with a small pin vise.
Vacuum-secondary carbs rely on velocity through the primary side of the carburetor to open the secondaries. Always make sure there is a good seal between the diaphragm housing and the carb to ensure proper vacuum-secondary operation.
The Case of the Missing 12 HP
Tim Hastman; via CarCraft.com: I have built a '90 Mustang GT with the following mods: '82 351 Windsor, Lunati hydraulic roller with 0.536-inch intake and exhaust lift, 218 degrees duration at 0.050 inch with 112 lobe separation, Performer RPM Air Gap, BBK 131/44 long-tube headers, 9.4:1 compression, Edelbrock Performer heads, Mallory Hyfire VI ignition, 3.73 gears, and a five-speed. The car is driven daily. My question concerns the carb. Initially, I had a 4150-style 650-cfm carb with a Proform body that made 316 rear-wheel horsepower (rwhp). To help driveability, especially in colder weather, I installed a new 770-cfm Street Avenger with an electric choke and vacuum secondaries. The car drives better, but the performance has suffered to 304 rwhp. Both carbs were tuned on a Mustang dyno with a wideband O2. The main question is where did the horsepower go? I expected a slight power loss, but not that much. Also, both carbs suffer from an off-idle sneeze through the carb when cold. Would having vacuum advance help? I just ordered the new Crane distributor for it. I think the heads are a little small. How much horsepower would I gain with AFR 185s?
On sticky tires, the car ran an uncorrected 13.04 with the old carb and 13.54 with the new carb, with a 1.98 60-foot time. Thanks, Car Craft rocks!
Jeff Smith: Let's deal with the loss of horsepower first, Tim. Your Proform-bodied 650 is a mechanical-secondary carb, which means you have a mechanical link directly to the secondary side of the carburetor. It's also sized to produce excellent air speed through the carb, and it was obviously not a restriction for max power. There's nothing really wrong with the Avenger carb. The main difference between the two carbs besides the airflow is that the Avenger is a vacuum-secondary carburetor. It is only slightly different from the classic 3310 780-cfm carb from the '60s. The biggest difference is that the Avenger uses straight-leg boosters instead of the 3310's drop-leg boosters, which some contend are of a superior design. But we digress.
The Avenger comes with a plain spring in the secondary-vacuum diaphragm, which the Holley catalog lists as a medium-strength spring. Just changing to the purple or perhaps the longer yellow spring (there are two yellow springs in the Holley spring kit-one long and one short) should bring the secondary throttle blades all the way open. This should restore the lost 12 hp. One advantage to the Avenger is that it comes with a Quick Change, vacuum-diaphragm housing cover, which makes changing the springs much easier. We would try increasingly lighter springs until the engine sags or bogs when you hit WOT. Holley's own chart lists the plain spring not completely opening until 8,160 rpm on a 350ci engine, while the yellow spring is listed as hitting full opening at 5,750 and the purple spring at 6,950 rpm.