I have a 71 f100 p-up 302 (stock)with factory 2- barrel carb with 3-speed. The only thing that is not stock is a set of headers. My problem is that out of the blue it has started backfiring very loudly on deceleration. I mean loudly. I rebuilt the carb and checked the points and plugs. The truck idles smooth and accelerates smoothly as well. Could slack in the timing chain cause this?