You have to check each valve stem & tappet clearance only this.Following the firing order 1-5-3-6-2-4 you have to put each piston in compression stroke(means two valves closed) and in this moment you can watch and turn with your finger the two tappets.Checked the piston number 1,turn clockwise 120 degrees(90+30) now you have the piston number 5 in compression stroke,turn again 120 degrees now you have the piston number 3,turn more 120 degrees you have the piston number 6 in compression stroke,same with the piston number 2 and 4.Normally backfire is caused because the valve stem and tappet clearance are not adjusted.Good luck.