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There are several simple modifications JET recommends to upgrade the classic Holley 4412 500 cfm two-barrel carb to make it race ready.

There are several simple modifications JET recommends to upgrade the classic Holley 4412 500 cfm two-barrel carb to make it race ready.

JET Performance Reveals a Few 4412 Carb Techniques

Words And Photos: Jeff Smith

Two-barrel carburetors don’t have to be the kiss of death for power. With the right modifications, even rules that forbid airflow modifications can still be used to your advantage. One of the most popular fuel mixers for these classes is the 500 cfm Holley 4412 carburetor. We talked with JET Performance Fuel Systems’ carb expert Johnnie Valdez, who offered to share a few basic tweaks they perform on these undersized carburetors to improve their performance.

Our carburetor, though off a restricted class car, was essentially right out of the box. JET suggested changes to the emulsion holes in the metering block (arrows) and also increasing the size of the power valve channel restrictors (PVCR).

Our carburetor, though off a restricted class car, was essentially right out of the box. JET suggested changes to the emulsion holes in the metering block (arrows) and also increasing the size of the power valve channel restrictors (PVCR).

These modifications are legal under most class rules since most sanctioning bodies only regulate modifications regarding airflow. The tuning tips in this quick story are all intended to improve the carburetor’s fuel curve. Of course, they didn’t share all their secrets — no decent carburetor tuner ever will — but even these simple modifications will help the fuel curve compared to an out-of-the-box carburetor.

The 500 cfm Holley 4412 is basically the front half of the classic 3310 750 cfm Holley carburetor with 1 3/8-inch venturis and 1 11/16-inch throttle bore diameters. Part of the flow discrepancy (half of 750 cfm is 375 cfm, not 500) is because Holley uses a test depression of 1.5 inches of mercury for its four-barrel carbs, while bumping this to 3 inches of mercury for its two-barrel versions. This is because the two-barrels should pull more vacuum at WOT if viewed as a flow restriction. Regardless of ratings, the 4412 can be tuned much like a typical Holley four-barrel.

Since we will be feeding a hungry small-block with only one needle and seat, one of the first things JET does is increase the stock 0.097-inch diameter version with a larger 0.120-inch assembly to increase fuel flow with the same fuel pressure. JET also replaces the stock brass float with an updated nitrophyl float wedge to maintain a higher fuel level in the corners. That may not sound like a big deal, but a stock float can push the inlet closed under high lateral g-forces, which is not desirable. A wedge-style float allows the fuel to stack in the right hand side of the fuel bowl while minimizing the effect on the needle and seat.

Holley outfits the 4412 with a 50cc accelerator pump body that JET has found is unnecessary. Valdez replaces the larger pump with the smaller 30cc pump found on most Holley four-barrel carburetors.

Holley outfits the 4412 with a 50cc accelerator pump body that JET has found is unnecessary. Valdez replaces the larger pump with the smaller 30cc pump found on most Holley four-barrel carburetors.

Valdez also suggested the stock Holley 50cc accelerator pump is overkill for this application, so they regularly replace it with a stock four-barrel style 30cc pump.

Moving to the metering block, JET will evaluate the engine size, breathing capacity, compression, and cam timing variables and recommend changes to both the emulsion circuits and the power valve channel restrictor (PVCR) orifices. Valdez recommends the power valve be retained to allow the engine to run crisper at part throttle, which is a critical yet often overlooked factor for circle track carburetors.

Certain Holley 4412 throttle plates come drilled with a pair of 0.093-inch holes to allow additional bypass air at idle. Because these engines are often equipped with long duration camshafts, JET recommends opening these holes to 0.173-inch. The main advantage is to more fully close the throttle blades at idle so that they barely uncover the idle transfer slot. This drastically improves idle quality and prevents pulling excess fuel at idle. A secondary benefit is the larger holes tend to prevent pulling excessively rich air-fuel ratios on deceleration as the car enters the corner.

Holley places stock 0.093-inch idle bypass holes (arrows) in the “215” throttle blades. JET has determined these are undersized for most circle track cams so they drill them out to a much larger 0.173-inch. This relationship is shown with the two transfer punches. This allows you to close the throttle blades to the proper position relative to the idle transfer slot.

Holley places stock 0.093-inch idle bypass holes (arrows) in the “215” throttle blades. JET has determined these are undersized for most circle track cams so they drill them out to a much larger 0.173-inch. This relationship is shown with the two transfer punches. This allows you to close the throttle blades to the proper position relative to the idle transfer slot.

We’ve only touched on a few simple changes, but if you have a carburetor you think needs some professional help, JET offers three levels of modifications, with Stages 1 and 2 making all of the changes mentioned here plus several more. Stage 3 mods increase the venturi diameter for classes where airflow modifications are legal. Check out JET’s website for more details or give them a call on their Tech Line. Just because it’s a two-barrel doesn’t mean it can’t be optimized. All you have to know is all the right moves to make to help that carburetor work to its full ability.

Source

Jet Performance Products
jetchip.com


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  • July 4, 2017 at 6:59 pm

    Can the carburators be sent in for modifications to optimize them? I race a dirt track factory and need all the help I can get. Also, what is the cost and turn around to get it back?

    Thanks,
    Daniel

    Reply

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