Burnt valves? Stuck valves? Is it consistant, like a timing issue, or inconsistant? Does it do it all the time, or when the motor is warm/cold...

j

 

timing chain may have jumped a tooth. does it get any better the more the motor revs up? is the carb adjusted properly? Holleys take some time to get just right. if its backfiring then it sounds like a timing issue.

 

and welcome to the forum!

 

im having a simular problem im slowly narrowing it down though my vac adv needs adjusted ive backed it out some and solved the backfire still need to turn it out more its hesitating part throttle anywho theres alittle allen head screw inside of the vac adv where the hose hooks onto try adjusting it some ,....... it helped mine after i added new intake , carb ,redid the heads , and everything else was new

 

if its a 71 then chances are your chain has stretched a bit so i would try and advance the timing about 2 deg, and also check the chain and see how much its stretched