This is where the idle-feed restrictors are located on most 4150-style Holley primary mete
Before we dive into your Holley, we have to assume that your 750 Holley is a vacuum-secondary 3310-style carb with idle-mixture screws in the primary side only. If your carb is different, our suggestions also will work with any of the many variations of the Holley 750 family. The photo we've included will show you where you need to work on a typical Holley primary metering block to accomplish the modification. Because this is a classic generic performance carburetor, Holley had to assume that it would be used on a variety of engine combinations. Therefore, the idle circuit was purposefully designed to be rich. This covers up a multitude of potential problems, but also demands tuning if your combination prefers a leaner circuit
The key point to work with is the idle-feed restriction hole in the Holley metering block. This restrictor is generally between 0.032 and 0.035 inch in diameter and is large enough to feed a 454 or an even bigger engine. To reduce the amount of fuel delivered by the transfer slot, the simple fix is merely to stick a small strand of copper electrical wire in each of the two idle-feed restrictors in the metering block. This will reduce the amount of fuel delivered. Let's do a quick example: Say you find a piece of 0.015-inch wire. How much will this reduce the flow area of the idle-feed restrictor? Way back in high school, we learned that the area of a circle is found using the formula pi x radius x radius. With an idle-feed restrictor diameter of 0.0350 inch, this produces 3.1417 x 0.0175 x 0.0175 = 0.000962 square inches of area. If we used 0.015-inch-diameter wire, this has an area of 0.000176 square inches. Subtracting the wire area from the large hole creates 0.000786 square inches of area that is 82 percent smaller. This means we've reduced the area of the idle-feed restrictor by 18 percent. This may actually be too much, but it is worth a try.
Once you have the carb reassembled with the idle-feed wire restrictors in place, readjust the idle-mixture screws. First, run both screws all the way in and then open the screws two full turns. Make sure the engine is at normal operating temperature before final adjustments. If you have access to an Innovate Motorsports wide-band oxygen-sensor tool, this will help you dial in the ideal air/fuel ratio. If not, spend at least five minutes working on the best (leanest) idle-mixture screw setting. You will know when you get too lean because the engine will surge and stumble at part-throttle cruise or light acceleration. When that happens, turn the idle-mixture screws out (counterclockwise) a little bit and experiment. You should be able to adjust the idle mixture to around 14.0:1 air/fuel ratio and the engine will be happy. This will improve your fuel mileage.
This same kind of trick can be accomplished on a Q-jet, but, unfortunately, the idle-feed restrictor is located at the bottom of a long tube that is staked into the main body of the carb. It can be removed, but then you will need to plug the bottom of the orifice with epoxy and drill it with a much smaller idle-feed restrictor, assuming that it is too large to begin with. Where this gets complicated with a Q-jet is that there are about a million different Q-jet applications and each one has a different idle-feed restrictor size-some too small, some too big, some just right. This is much more difficult a task to perform unless you have a lot of carburetor experience, and will also require some deft fabrication skills. We suggest sticking to the Holley-it's much easier to modify.
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Innovate Motorsports; Irvine, CA; 949/502-8400; innovatemotorsports.com
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