Check the crankcase oil level to see if it is above full and smells like gas. If it is your fuel pump is leaking. This will cause an engine to stop while running and will also keep it from starting.

If no gas is in the oil try to start it as usual. If it won't start spray some carb carb cleaner from a spray can into the carb with the choke held open. If the choke is closed this won't work. Don't pour gas into the carb it could backfire and start a huge fire.

If it starts with the carb cleaner and runs check that the choke plate opens after a few minutes as Asylum says above. However, if only the automatic choke were your problem it would start easily when cold.

If the choke plate opens ok and the engine stops after running a short time take off the air cleaner and move the throttle quickly a couple of times to see if it sprays gas into the venturi. If not you have a fuel supply problem.

My first guess would be a bad condenser because I have had all these problems with condensers, but I think a '76 has HEI so that theory is no good if it has HEI. If you have a condenser, change it.

Be sure to let us know what you find out if you solve the problem.

'31 Chevy 1 1/2 ton,
'42 GMC 2 1/2 ton 6x4,
'52 GMC 2 1/2 ton 6x6,
'53 GMC 5 ton