2009 AquIla 250 Not starting

Need some help with this. My buddy just bought a 2009 Aquila 250 with the understanding that the bike did not run. The previous owners stated he did not turn off the petcock valve when he stored it for the winter. My buddy bought the bike, drained the gas, changed the oil, spark plugs, and the bike is still not starting. Bike cranks, but not starting. We took off the air filter and tried spraying starting fluid up there, nothing. Made sure the bike is sparking and it is. We are at a loss with this. Any ideas?

Also side note, when it is cranking and a put throttle on it, it starts to back fire.