This was probaly re-hashed,but when I bought my 76 Sporty,it wasnt running on the rear either. Turned out the OEM breaker-less ignition was junk ( advance springs-weights were clapped out) thus causing miss-fires and massive carbon build-up on the rear valves/seats. Took the head off,and cleaned the valves-seats,chamber etc ( the valves werent seating,causing compression loss)....replaced the ignition with a new aftermarket electronic unit....runs like a top.