Backfiring
 

Hey guys, a newbie here. Just recently bought me my 1st. mazda truck. A 91' B2200 straight carb..Problem is, it backfires when shifting (down or up)..I changed spark plugs, wires. Was thinking of doing the rotor next, IDK..Maybe I can have some insight into what i'm doing or lack there off.. Appreciate all/any information I can. Thanks for looking..