Do you have gas? Is the small vacuum hose hooked to the front of the petcock? Whats the air filter look like? feuel tank vent line pinched? What were you dong when the bike backfired? Have you done work to it recently?
Has fuel vent hose clear all hoses to petcock secure about 1000 miles ago did plugs and front rear brakes about a week ago
Diaphram in petcock may have failed. Verify fuel flow. I would remove petcock fuel line, run a scrap to a container and crank the bike. When mine was going bad, sometimes the act of removing and reinstalling the gas line would get it going again. If gas is good, next verify spark. Bike may be flooded at this point from repeated attempts if you have had gas.
I don't think that it though....its just a vent for crankcase. open to atmosphere wouldn't matter. If not gas, I would be looking at the kill switch area, ECM area. Both have been known to break wires loose. Good luck!
I am getting my bike out of shop today for same problem. The problem was a broken ground wire at plug on ECM. This is the second replacement harness for same wire breaking. The wire is a green wire with yellow stripe. It breaks 1/2" from plug.