i have an 87 softail i just had the motor rebuilt at the dealership on the second day i had it back after about 15 hours of riding the bike backfired horribly power was lost and the bike shutdown i turned the ignition off gave it a second started it again the bike road great for about1/4 to 1/2 mile and then repeated the actions again back fire loss of power and electrical sent it back to the dealer they say they cant find a problem can someone please help me.