running rough hard to start

I have a 76 kz 900 and ever since i bought it it has been running rough and hard to start. I cleaned the carbs but it back fires sometimes and when it does run i can ride it for about 2 miles and it will die and not want to start back up for awhile. I replaced the plugs and wires and cleaned the carbs what else could the problem be? The bike sat for over a year in a garage too. Thank you for the feedback.