My son has a 77 F150. It has a 351M with a mild cam, 4bbl, headers and dual exhaust. The motor has about 135K miles. It has started backfiring through the exhaust and seems to be limited to the left side. It starts and idles fine. It just pops and backfires under acceleration and at a steady cruise. It quits when you let off the throttle. We have changed the plugs and put on new wires. The timing is at 12 degrees BTC. Haven't found any vacuum leaks. All the engine mods were done over a year ago and it ran fine until recently. He is going to put a vacuum gauge on it tomorrow and see if that tells us anything. Any ideas or solutions?

 

its sounds like some exhaust valves are sticking