backfire

1: remove the distributor cap and turn the engine in one direction until the rotor starts to move. it doesnt matter which direction. then turn the engine in the opposite direction again until the rotor starts to move. if the timing chain is in good condition then the rotor should move with little movement of the crank. if you have a substantial crank movement, more than a few degrees, then replace the timing chain.

2: bring the engine to tdc on the #1 cylinder on it compression stroke, and check the rotor position and firing order.

3: check your timing.

once you have completed these tasks, then run a compression check. i am betting that you will find a low cylinder or two, and with the fact that you hear air coming from the carb, you likely have burnt a valve.

excellent, what was the problem? rather what cured the problem?