Starting backwards..definitely a possibility. Usually caused by
off timing. Main cause of teeth chipped off ring gear. Not good.
I've always thought the flapper valve in the exhaust was a little hinkey,but
who knows. Internet echoes say Yamaha used same system at some
point in time. My theory is anything located in the exhaust will
eventually fail. Exhaust gas temp probes always failing.
Last gossip I heard is if your cat gets oiled up it is toast.