Dear Brit-Iron,
The tried and true remedy for an immediate kick-back fire is to go shut off the gas tap, go to wide open throttle and kick start like mad. Thereby sucking the fire into the engine (where it belongs). this has worked well for me on several occasions (DAMHIK). But nerve is required. A back-up plan is to have a blanket or dry cloth handy to smother the fire. (Having turned off the gas tap.) Next level of back-up is to have the proper kind of fire extinguisher for gasoline fires handy.
But before the fire happens, it is best to have the fuel lines, taps, floats, carb controls, ignition advance/retard/timing set properly so that the combustion occurs where and when it should.
What is wrong with that group of motorheads? Maybe just panic?
Al Johnson