Backfiring through the carb would be a timing issue, possibly a worn distributor. A timing chain jumping a tooth would do the same thing--fine with no load, but load the motor and it backfires through the carb. You can check to see if the chain is loose by removing the distributor cap, put a wrench on the bolt in the center of the crank pully, and line up the timing mark on the pulley. Then move the pulley back and forth until the rotor moves at the distributor. The pulley should only move a couple of degrees each way before the rotor turns, if you can rotate the pulley several degrees before the rotor turns, this may be a loose chain. Just one more variable to eliminate.