Fat Idle Fix
You might not realize it, but a street engine spends a majority of its life between idle and cruise mode. This activity is all controlled by the carburetor's idle circuit. Within this circuit, the combination of the idle-feed restrictor, the idle-air bleed, and the idle-mixture screw establish the maximum amount of fuel delivered by the idle circuit. Of the two fixed restrictors, the idle feed is the most important. A typical Holley 750 double-pumper 4150-style carburetor is generally fitted with an idle-feed restrictor of 0.034 inch, an 850 will have a slightly larger 0.037 inch, and the idle-air bleeds will measure between 0.067 and 0.070 inch. Most Dominators are fitted with much larger 0.041-inch idle-feed restrictors. This may not seem like much, but you would have to reduce the 0.041 restrictor by 31 percent to equal the area of the 0.034 restrictor. If you want to do the math, remember that area is Pi (3.1417) times the radius squared. This means the
Dominator's idle circuit has a flow area almost one-third larger than a typical 750 4150-style carburetor, which is already a somewhat rich idle circuit for many street engines. The Dominator is so fat because it's intended to be used on competition engines with minimal idle vacuum. On a street engine, the cam will be less aggressive with more idle vacuum and will "pull" more fuel from the idle circuit. This means the idle-feed restrictor must be reduced to return some control over the idle circuit. For example, we offered Murphy the challenge to tune a theoretical 450-460ci street engine with a single-plane intake and a cam with 250 degrees of duration (at 0.050) in a relatively heavy 3,600-pound car with a 2,800-stall converter and 3.73 gears. Murphy suggested reducing the idle-feed restricted down to around the 0.037-inch size while the idle-air bleed should be opened up to around 0.076 inch. Notice he's matching idle-feed and idle-air-bleed changes. Both of these steps are intended to lean out the idle circuit in an attempt to improve off-idle driveability. This can then be a starting point for additional tuning when accompanied with input from a wideband-style air/fuel ratio meter like the one from Innovate.
Holley HP carburetors and Dominators move the idle-feed restrictor from its normal locations (A) up to the top of the idle circuit (B). This is done to make the idle circuit more responsive to changes. If you decide to move the restrictor to the top location, drill the original one so it's not a restriction. | 
A quick way to reduce the size of a large idle-feed restrictor is to use a small-diameter wire bent in a U-shape. The larger the wire, the more area is displaced. For a 0.041-inch diameter idle-feed restrictor, a 0.015-inch diameter wire reduces the area by 13.5 percent. |