I have a '69 Chevelle SS and recently purchased it. I have noticed that when I step on it and let off the gas, it backfires farely often. The more gas I give it, the harder it backfires. Before I got the car, the guy who owned it before me got the car when it no longer ran. He bought a carb kit and it ran fine after that. I am presuming that the carb needs either tuning or replacement. Its a Rochester Quadrajet.