help pleaseeee

me and a few buddies just tried gettin my duster runnin and were pretty confused. heres the deal when the cars in neutral i can get on and punch the gas as fast as i want and it will run mint and smooth but wen i put it in gear it will run horrible. it will backfire left and right no matter how much throttle is on, and as soon as i punch it it backfires and stalls. the only thing we can tink of is the timing chain beacuse we played with the timing and got nothing better
thanks for the help