It's usually because there's too much timing for the fuel available, but some of the 70's era carbs had an anti-dieseling solenoid that would drop the idle speed below normal to make sure the engine stopped.
A bad, stuck or dirty float needle inside the carburetor.
not enough octane in your petrol. To stop the run on, turn your engine off while it is in Drive. Then switch to Park
bad seal in between your intke manifold and carburetor
your float is stuck in the carburetor. Clean it with carb cleaner.
Usually a stuck float mechanism.
A misfire will cause flames to come out of the carburettor
the jets are inside the carburetor. They look like brass pipes, they can be unscrewed. The jets have to be clean (the hole has to run all the way through the jet) in order for the engine to run.
run the engine out of gas!
The motor simply wont get enough fuel to run properly.
carburetor or spark plugs
The float valve in the carburetor is not working properly. You will need to remove the carburetor and clean it or replace it.
your main jet is plugged