Actually great.
Did you put the air cleaner back on?
Do you have manifold heat?
Did you try pulling the choke out 1/3 to 1/2 and moving it again?
Did you read the vacuum from the vacuum advance port?
This instance is not caused by fuel supply to the carb so forget about everything up stream of there.
Things that happened between running good and carb backfire
(Assuming)you disconnected the vacuum gauge and re connected the vacuum advance.
1. the engine was warming up.
2. the intake air dropped below freezing
3. you put a load on the engine and opened the throttle valve
4. the engine ran smooth when the load was removed
5. the engine ran smoothly when the butterfly is opened slowly
