WTF! motor is backfiring

[olyfiveoh]

I have a 90 GT and latley my motor has been backfiring. it happens either when i shift into 2nd or into 3rd. if i rev the motor up before i engage into 2nd gear it dosent do it. but if i put the clutch in, put it in 2nd, let the clutch out and then give it gas it usually backfires. what would cause this?