Side by side next to a standard...
Side by side next to a standard Nitrophyl float, the difference in profile is readily apparent. It is easy to imagine how the modified float will be less likely to be upset by fuel slosh to either side.
The first modification was to the carburetor floats. Center-hung Holley floats were initially designed with acceleration at the forefront, and when g-forces are active during cornering, the fuel in the bowls responds by sloshing. The modification here is to take the standard Holley Nitrophyl floats and shave the material at the corners, converting the float from a rectangular profile to a trapezoid, resembling a triangle. What this does is concentrate the center of buoyancy away from the outer extremities and moves it to the center of the bowl. Thus, fuel sloshing left or right will have a lesser effect on the float position. To seal the pores of the Nitrophyl float material, the cutaway surface was coated with 5-minute epoxy. The epoxy not only seals, but provides a small amount of additional mass that dampens the float's response.
Our primary throttle plates...
Our primary throttle plates were equipped with air bypass holes. With an objective of increasing the idle throttle angle slightly to make the transfer slot more active for improved light throttle response, these were replaced with a new set of solid throttle plates.
Next on the hit list were the needle and seat assemblies. Here the internal restriction was enlarged from the stock .097-inch orifice to .120 inch. The actual dimensional change might seem small with an increase of only .023 inch in diameter, but this represents a change of 53 percent in cross-sectional area open to fuel flow. The higher flowing needle and seat assembly makes the carb much more sensitive to float inputs, allowing the bowls to fill faster once the needle separates from the seat.
The fuel in the bowl is only useful to the engine if it reaches the jets, and under very hard acceleration or braking, this can become difficult. As a nod to fuel control, both the front and rear jets were fitted with short jet extensions, which place the fuel pickup point virtually dead center in the bowls. This strategy provides the best opportunity for the jets to find fuel whether accelerating or braking.
When replacing throttle butterflies...
When replacing throttle butterflies it is important to center them precisely in the bores to a fully closed position, using light pressure on the leading edge.
Longer bowl vents were also added to the main body, making it less likely for fuel to escape the carb while the vehicle is pressed into extreme maneuvers. Keep in mind when contemplating longer vents that there needs to be adequate clearance to the air cleaner lid, otherwise the vents will not vent, defeating their purpose.
Addressing the light throttle and transition irregularities, the first item Bretzing keyed on was the drilled primary throttle plates. Drilling the plates has long been a popular modification to allow additional air into the engine at idle, particularly with long duration cams. While this can work well, it is a primitive approach, since this allows no metering activity to take place at the throttle plate holes. The holes effectively bypass the normal idle circuit, acting as a measured vacuum leak. While the holes allow less idle throttle angle, if the air bypassed is in excess to the engine's requirements, the result can be compromised low-speed metering, particularly in the transition from the idle circuit to the transfer slot. On our carb it was determined that the low-speed, low-throttle driveability could be improved by increasing the throttle angle at idle slightly, making the transfer partially active at idle. The theory here is that transition to the transfer slot portion of the idle circuit will be more active, covering the lean hole at low throttle inputs. To that end, the throttle plates were replaced with standard butterflies without holes.