bad crank sensor, fixed car and new problem
 

my FD broke down the other day. I was just driving it and the engine died and would crank but not start back up. I towed it back to my shop and later we diagnosed it as a bad crank sensor. one of them was reading no continuity. I swaped in a spare one from a spare motor and the car fired rite up. So yay figured the problem was fixed. Today i went to test drive the car and at about 3000-3500rpm the car will backfire. At idle if you give it gas the car will backfire also. car ran fine prior to the crank angle sensor problem and i dont have a check engine light. Does anyone have ideas on what it could be or what i can check out? need some help. Thanks