Put a vacuum gauge on the intake- - - -full vacuum, not "ported". If the exhaust is restricted (I guarantee it is!) the vacuum will drop to virtually nothing and the engine will try to stall as soon as you get a few RPM above idle. The backfiring is caused by the exhaust leaving the engine the only way it can- - - -through the intake! "Been there- - - -done that"- - - -many times!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!