I didn't see you mention your fuel.

The amount of oil matters as does the amount of nitro.

Cold plugs are needed for high nitro fuel. Hot plugs are needed for cold burning fuel (need to keep their own heat). When a plug doesn't match the heat of combustion AND the engine is a sensitive one to timing you might have needle setting problems.

BTW, the heat range of the plug often changes when the combustion starts. That changes the timing of the event. Change the timing and they often act up.

What is your fuel?

Also, have you pulled the head of that engine? Did it have any headgaskets? Lots of the old ones did. Some would have as many as 3. They adjusted the mfg tolerances of the day with headgaskets sometimes. Changing the number of headgaskets changes the compression which also changes the timing.