sputtering,backfiring

[Mikey81xsbobber]

greetings, my 81's been running fine.then all of a sudden,when i get into 3rd,4th,and 5th gear it begins to sputter,lose power,and backfires..recently changed the plugs..any ideas would be greatly thankful.. thank you.

 

[XSLeo]

81 has the TCI. The TCI is very voltage sensitive. It could be low voltage caused by a weak old battery or a fault in the charging system. Or maybe a fault in the TCI box. Often just resoldering the joints in the TCI box fixes the TCI trouble.
New brushes in the alternator can fix the charging.
What battery voltage do you get at idle and 2500 rpm?
Leo