I had one the other day. My Align .91 is very sensitive to being hydro locked with fuel. I pinch the fuel line when shutting down, and I pinch the feed line when fueling, but I often have the starter crap out because the motor is fuel locked.
So I turn the bird with the muffler side lying down and turn the motor over by hand with the wand so the extra fuel runs into the muffler. Usually this fuel does not cause a problem, but this time it exploded when I tried to start the motor.
A close inspection of the fuel system showed that the pressure line from the muffler to the tank had split open just before the nipple.
So, just a heads up, if you have a backfire, check the pressure line. If you turn the bird on it's side to drain the fuel, also hold the bird nose up to get the excess out of the muffler.
Hope this helps.
"Sweet dreams and flying machines in pieces on the ground." J. Taylor