Backfires are not usually a big deal. Most occur when fuel ignites when the intake valve is partially open and the spark plug fires the cylinder. Imagine this situation.... you crank the engine drawing fuel into each cylinder. The engine does not fire right away (i.e. cold start) and you let off the key. The engine stops just at the point of ignition to whatever cylinder, but the motor backs sightly due to compression kick back and the intake valve comes back open slightly igniting the fuel in the intake making the loud bang. I would crank the motor again, and if it replicates a back fire again, there are more serious problems, but my guess is that it will start just fine. Your timing may be a bit too far advanced too.... Sorry fot the loooong post.