Cold pops are usually lean, but could be TOO rich and unburned gas in the headers. Could be contaminated or old gas, and rarely plugs unless they get wet. I've never traced it to timing.
Worst case is probably a sticking exhaust valve.When an engine misfires, that A/F charge moves down the pipe but it's ready to burn. So if/when it sees a different ignition source, it catches fire and pops. You can feel/hear it in the tailpipe .
If it pops up near the header flange, then the header is probably sucking fresh air, tighten the screws and see what happens. In this case, as the engine warms up the popping changes to ticking
