The 1946 carb is designed to be used with EGR systems.
If you don't have one, the result will be an over-rich mixture in the lower-midrange (just above idle, like a 1/8 throttle). This is where the EGR puts the most exhaust into the intake, leaning out the resulting mix.
To tune an engine without EGR underneath this carb, you can use a lighter spring in the distributor's primary side. This will give earlier advance through the lower RPM range and offset the richness, giving more torque. There will then be a slight 'flat spot' in the spark advance from about 1800RPM to around 2200RPM, but chances are, you won't notice it when driving.
The main advantages of the 1946 are the 'active' design of the power valve and the larger venturi (1.75") as compared to most of the Mustang carbs (1.5" or 1.625" venturis are more common). While some claims are in these pages as to getting better MPG, that will only happen if you have the EGR running underneath this carb.
Hope this helps!
