I own the same sled with the similar problems , low end loss of power, backfiring sometimes, runs rough low end until about 5,000 to 5500 rpms then it straightens out but I have also experienced a loss of power top end sometimes top rpms would be 7000, and with wide open throttle rpms would sometimes drop to 6,000 like the sled was starving for fuel, I have taken the carbs off cleaned and reinstalled, adjusted balance of throttle plates ,with little difference in performance
found two solenoids on each carb, I have disconnected all solenoids found this makes in run rougher and sets the diagnostic light flashing disconected the two lower solenoids ran rough backifred even worse, I then disconencted the two upper solenoids and top end is better than it has ever been,8,000 rpms now versus 7,000, and no loss of power at all, backfiring is gone , but still have low end rough running until about 5,000 rpms, just found this yesterday, and I am still working on the problem
better but not fixed right,by disconnecting stuff, so if anyone has any info it would be appreciated or maybe a website with diagnostic info, hints anything,