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ProForm Main Body Installation, continued.

The main body kit includes jets and recommending jetting. We suggest simply bumping your primary and secondary jets up three sizes as a safe baseline starting point. We originally had 71 jets in the primary and stepped up to 74. Chance are you'll end up very close or exactly at your previous jetting, assuming you were tuned for optimal power to begin with.

On the secondary metering block we took the opportunity to install jet extensions (the brass standoffs.) The purpose of these extensions is to place the jet further into the fuel bowl so that they do not become uncovered as the fuel sloshed back in the bowl upon sudden acceleration, which can result in a lean bog. We recommend these for anyone drag racing a carbureted car.

Since this carburetor had been sitting on the shelf for a couple years we changed out the power valve, as the rubber diaphragm becomes brittle.

The supplied nonstick gaskets are set on the metering block. Pay attention to the self-locating holes.

A tip for those frequently performing jet changes with the carburetor on the engine. File off some material above the large vacuum port at the rear of the carburetor base plate. This allows the secondary bowl and metering block to be more easily removed.

Install the metering blocks on to the new main body.

Another worthwhile upgrade while your Holley is apart is to replace the brass fuel level floats with these Nitrophyl floats, available from Holley. The brass tends to develop pinholes in the brazing, eventually filling with fuel and poor fuel level control in the bowl.

The needle and seat assembly also needs to be removed to access the floats.

Remove the two screws securing the float to the bowl.

Carefully pull the float straight out so as not to loose the pin and tiny spring.

The pin, spring and bracket must be transferred from the old float to the new one as shown.

You've transferred the mechanism correctly when the spring tension forces the float to the bottom of the bowl as shown here.

Carburetors that have been sitting dry for some time tend to develop deteriorated needles and seat seals. If you find fuel flows out of the vent tubes when you try to start the engine, this is surely the case. Buy new replacements at about $20 a pair.

When reinstalling the needle and seat assembly be sure to use two gaskets, one between the adjustment nut and bowl, and the other under the lock screw.

If you are using jet extensions in the secondary metering plate you'll need the notched version of the Nitrophyl float (left). In a pinch you can file notches in a standard Nitrophyl float.

Preset the float adjustment. Turn the bowl upside down and let the float hang, it should should be at the bottom of the sight hole in the bowl. Adjustments are made by turning the needle-seat adjustment nut (clockwise to lower the float and fuel level.)

Finish up the installation by attaching the bowls and metering blocks to the main body.

The ProForm comes pre-installed with primary and secondary pump discharge nozzles. Unfortunately they are not marked with a size (probably .028 in.) We ditched them for the .035 in. which work best for us.

Accelerator pump discharge nozzles are sized in thousandths of an inch (stamped on the body.) The varying styles, from left to right, standard, with discharge tube for better flow direction, and anti-siphon. They all work the same in our experience.

There you have it, a fresh 650cfm double pumper. With the new ProForm main body there is no hint this is a 1980's era 4150.
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Power Valve Selection

Holley power valves serve to add fuel when engine load is moderate or high. When engine vacuum drops to the rating of the power valve the valve opens and allows fuel through the Power Valve Restriction channel in the metering block. The power valves are stamped on their face with the vacuum rating, in inches.

The best way to select a power valve is to hook a vacuum gauge to your engine and drive around at part throttle. As you gradually step into the accelerator note the lowest point the gauge reaches before vacuum begins to rise again. Select a power valve with a rating 1 or 1.5 greater than this number. Alternatively a rule of thumb is to select a power valve with a rating of half your engine idle vacuum reading.

Many racers choose to block the power valve due to various reasons, including big cams with engine vacuum so low the valve open unpredictably. A power valve block can be installed however jetting must be bumped up several jet sizes to compensate.


Bits and Pieces

The venerable Holley double pumper has been around for half a century, and shows no signs of retiring any time soon. As such, there are plenty of companies with products to enhance its' performance. Here are a couple to ponder.

Moroso offers this pump discharge nozzle screw. The radiused allen head claims to offer less turbulence and the larger diameter fuel slot is apparently repositioned to allow fuel a less restrictive shot to the discharge nozzle. At $8 maybe it's worth a try, though you'd be hard pressed to see any measurable gains.













Specialty Auto Parts U.S.A, Inc. / Proform
Post Office Box 306
Roseville, MI 48066

1801 Russellville Rd.
P.O. Box 10360
Bowling Green, KY 42102-7360



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