A backfire damages the intake box that controls the fuel pump and fuel
injection.  

Try this:  turn on the ignition, then pull the 3"x3" plastic cover off the
potentiometer of the intake box.  

GENTLY move the wiper and note if the fuel pump starts to run, and shuts off
when you let the wiper return to the stop.

The air vane in the intake box gets warped by a backfire, and then causes
problems.  A bad intake valve is more of a rarity.  Do a compression check
before pullilng the head.

Rgds, Ralph Ricks   81vangon, 2 914's, etc.