Just started my 78 shovel and got blasted with a series of very loud backfires, she stalled and gave a puff of smoke out of the air filter. Bike was running like a dream 2 days ago. It will start but keeps backfiring. Any suggestions?

Condensor? if you still have points ignition, or points, ignition timimg, start by setting plug gap, timing. Check all wiring terminals. Won`t be anything to serious, swap ignition with a known working one if you get stuck.

as houli say the condensor if you have points ign,    if it is you will have to clean the points or replace  them  +  best replace the spark plugs  as well

 Thanks for the tips guys. I will be onto it asap. Cheers.