Check what? A simple test that could be worth 10 hp is to merely checkto make sure your carb linkage actually achieves wide open throttle(WOT). Have your buddy jump in the car, and with the engine off (do wehave to tell you that?), have him hit the throttle while you make surethe carb opens fully. In addition, for older musclecars with solidlinkage, you could make a slick link with spherical rod ends at eitherend for more precise operation.
One common problem with Holley carbs is gorilla-fingered tuners.Overtightening the bowl screws will often warp the metering block,creating a vacuum leak that is difficult to diagnose. The easy way tocheck this is by placing a straightedge across the metering-block face.If you see light, the block will need to be milled, which should be leftto a professional carb rebuilding company like The Carb Shop(www.customcarbs.com).
Fuel pressure is a key element to ensure solid engine performance.Holley makes a small fuel-pressure gauge (PN 26-500, $24) designed toplug directly into the fuel line at the carb. You could also rig up alength of copper line and place the gauge up on the cowl temporarily tocheck fuel pressure at WOT. The fuel pump is taxed most heavily at thetop of high gear on a full-throttle dragstrip pass. Make sure you haveat least 4 to 5 psi of fuel pressure at that point. If not, startlooking at improving your fuel delivery system.
Speaking of bowl screws, ARP offers stainless steel bowl-screw fastenersfor 4150-style Holley carbs (PN 400-0310) and shorter screws for therear bowls on 4160 Holleys (PN 400-0311, $24). You can pick up eitherkit from Summit and add a little sparkle to your favorite fuel mixer.Or, if you prefer more practicality, ARP offers several types ofstainless carb-stud kits as well. The standard kit is shown here (PN400-2401, $16) and you can also pick up a stainless air-cleaner stud andnut (5/16 x 2.7 inches, PN 400-0302, $8).
Idle Speed Deeds
Big cams and low idle vacuum can make for an off-idle stumble that'soften tough to cure. The problem stems from cranking the primarythrottle blades open too far, which uncovers too much of theidle-transfer slot (arrow). When you lightly step on the throttle, theengine stumbles or sneezes because there's no additional fuel availableto help accelerate the car, since the transition slot is already in use.The trick is to close the primary throttle blades down to where thetransition slot is barely uncovered.
This step will create a too-low idle speed. One way to help that is tocrack open the secondary idle stop (located on the bottom of mostHolleys on the passenger side) a half-turn or so. If that doesn't help,you might try adding a PCV valve to the carb, which is also a calibratedair leak that will add a little speed. If all those tricks still don'thelp, you will have to resort to drilling a small hole in each primarythrottle blade on the leading edge. Start with a 1/8-inch hole in eachprimary throttle blade and work up one size at a time. It's a slowprocess, but that's the best cure for the big-cam off-idle stumble.