Your 1406 is a vacuum operated secondary carburetor. Air flow through the secondary circuit is controlled by a pair of air valves that are located in the secondary bores above the throttle blades. They are balanced against a counter weight and open to admit additional air flow only if there is enough air velocity (vacuum) to allow the proper operation of the secondary circuit. In all likelihood, your jetted rich, but we need a bit more info first.

A few questions to allow help us address the problem:

What year is your 305 and what type of car/truck are you working on?

Standard or automatic transmission? If automatic, do you have a stock stall converter?

Stock rear end?

Is your 305 stock? If not, what else has been done to the engine?

What manifold are you running?

Did you buy the car with the 1406 installed or did you replace a stock configuration?

Fill us in and let’s see what we can do for you!

Regards,
Glenn