Working on a 1982 Chevy c20 with a 5.7 liter 350 and a manual trans. Truck bought at auction so unknown history. Vehicle starts and runs but appears to pop back through carb once engine is warmed up when accelerating or holding at higher rpm. Timed using timing light and popping appears to happen even when vacuum advance not hooked up. Timing also appears to jump around a little (4-6 degrees) when checking with the timing light. When this happens you can hear it in one of the cylinders almost sounding like a miss. Wondering if anyone has came across this. Tried replacing the ignition cap in case spark was jumping, also put on china replacement quadrajet carb and new base gasket. Didn't fix the problem. Only thing I can think it may be is a sloppy timing chain is allowing the ignition timing to jump around and when accelerating or at higher rpm is causing the timing to move and is causing it to fire on or close to the intake stroke. Please reply with any suggestions.