1964 Chevrolet Corvette Coupe Small Block - 327 Dyno Tune - Dyno Drives

Float adjustment on this Holley carb model is external. Loosen the locknut and thread the needle valve inward to lower the floats, and outward to raise them.

The float level is correct when the fuel in the bowl just trickles past the sight-plug threads with the engine running.

We then moved to adjusting the idle mixture: in for leaner and out for richer. Adjust evenly on each of the screws for the best idle quality. Set the idle speed, then check the mixture again.

We followed the basic tuning with another dyno pull with the new carb, and found the secondary opening rate was too slow. This carb is fitted with vacuum secondaries, and the opening rate is controlled by the tension of the spring in the actuation diaphragm, with a lighter spring quickening the opening rate. With the Holley Quick-Change diaphragm housing, spring changes are quick and easy. This style diaphragm housing is available from Holley as an accessory for vacuum-secondary carbs.

With the new carb in place, Tom spun the Corvette against the dyno again and power was up significantly, showing 225 hp at the wheel, or 258 at the crank. Power was up over the entire rpm curve, but the dyno instruments recorded that the fuel mixture was still quite rich-about the same level as before.

Wide-open throttle performance and outright horsepower are important points of a dyno tune; but for a street car, tuning the part throttle mixture is just as important. An eddy-current dyno like the unit at Westech can set a part throttle load to simulate road conditions, and the air/fuel ratio can be read on the display or recorded.

The carburetor was disassembled for a jet change, moving from a 70/77 jet combination in the primary and secondary metering blocks, respectively, to 64/70.

A sharp dyno tune will include optimizing the part-throttle enrichment circuits, such as power valves in Holley carbs or metering rods and springs in Carters. Tuning the part-throttle mixture can result in smoother transitions from part throttle, and sharper throttle response.

This time, the computer readout showed the mixture was right on the mark. Power moved up accordingly, with 231 hp at the rear wheels, which is about 263 at the crank. The midrange torque increase was even more significant than the top-end numbers.