Is this a fresh rebuild? Has the carb run before?
The first thing I would do is put another carb on it that you know works on another motor. If that carb does the same thing it is not the carb. If it is flooded it can spit through the carb. But if you have flames shooting out of the carb it is a timing issue. If this motor is fresh and never ran before. I would check the marks on the timing gears. When you put a timing light on the motor you are verifying when the ignition is firing in relation to the crank. But if the timing chain is off the vavles will be in a different position than they should be. Open will cause fire to shoot up through the carb.
I don't believe you have to worry about melting a piston at idle when AF hits 14. At high rpms for extended periods of time yes.
One other thing. I had a problem with my car when I first set it up. It ran rich all the time. The vent in the gas tank was clogged and it built up pressure and was pushing fuel to the carb. Cleared the vent and things went back to normal.