got the problem figured out thanks to help from my neighbor. ignition switch is worn enough that its losing .5 volts across the switch. turns out the backfire was an effect of weak ignition and not the cause of the problem. after bypassing the ignition switch not only does the truck start fine, but it runs better and smoother than it has since ive owned it. I wouldn't have thought .5 volts would make that much difference but I guess when you only have 6 volts to start with you cant afford to lose any!