What is happening is called dieseling. The motor continues to run, despite that the ignition is turned off.

This is because the spark plugs, exhaust valves and general combustion chamber are hot enough that the fuel ignites on its own.

Since the carb supplies fuel as long as air is passing through it, it just continues to do its job.

The factories did a couple of things to prevent this.

1: Added a solenoid to kill the idle fuel.
2: In some cases added solenoid to close the throttle when the ignition is off.

What can you do?

The fuel kill circuit is not a DIY mod and has to be designed into the carb.

A solenoid can be adapted from cars that used a Weber knockoff. A Vega or a Pinto might be good donors.

Weber might have one as an option.

You can also drop your idle speed, lower is better when it comes to dieseling. More initial advance on the ignition timing will reduce the amount you need to open the throttle to get it to hold a certain speed. Most motors will take an extra 5? advance without any issues.

Make sure your motor is in good tune. Encrusted plugs and chamber deposits dont help.

Make sure the idle mixture is properly set.

If your getting really bad fuel economy, trace the issue down as well. Is your dist advance mechanism working correctly? The mechanical advance gets gummed up over time and the vacuum diaphragm gets torn.

Also make sure you have the dist advance chamber hooked to the ported vacuum line on the carb.

Start with idles speed and mixture. I would shoot for 750 RPM.

Kevin