If it backfires after you let go of switch
Try starting and let go of switch quickly
It sounds as if you have an issue with starting
Voltage. 1 -check your ignition switch as well as your distributor the distributor needs to be tight for ground 2- look at your wiring on coil make sure the wiring was put back in position
Is it a flathead engine ? If so check your timing
Since you had head off 3-make sure all of the ignition wires are tight especially on ignition switch 4- does it have an oil pressure or vacuum hooked to lp for safety switches
Lp units are supposed to have a vacuum switch or a oil pressure switch for safety in case unit gets a fire and engine dies it does not feed lp to engine 5 - if electric lp lock off put a meter or test light in it and see if it loses voltage if it is a vacuum lock off remove vacuum line and suck on it you should hear diaphragm close , if you get fuel it's bad
Anything I can do please let me know