Altitude has a direct effect on the operation of most carburetors. As the altitude increases, the air becomes less dense so a carburetor, originally calibrated at low altitude, delivers too much fuel and the engine runs richer. If the preceding tuning procedure is performed on an Edelbrock Performer Series carburetor, a proper high altitude calibration will result. If the vehicle was calibrated at lower altitude, however, and is to be driven at high altitude temporarily, it is not necessary to repeat the complete calibration procedure. Instead, use the rule of: “2% leaner per 1500 feet” and the Calibration Reference Chart for your model carburetor. For example, with a #1405 at baseline calibration (location #1 on the chart) and intended operation at 6000 ft. altitude, you would want 6000 divided by 1500 x 2% = 8% leaner calibration. That would be location #24 on the chart which would require only a rod and jet change.