If the engine is holding a steady speed with the governor rod disconnected from the carb then I'd say the carb is OK. If there was a problem then it wouldn't hold steady speed. The backfire does suggest it's a bit lean tho... I forget the exact procedure on adjusting the idle mixture... but basically I think you take the screw in until the engine stumbles then back it out again until it smoothes out.
The entire adjustment procedure is found on pages 65-66 in the Ford operators manual for your tractor.