[Backtobasics]

You might consider a sea foam treatment, to help clean valves and combustion chambers of carbon buildup. Carbon can cause pre igntion, so a sea foam treatment, letting it sit overnight, might clean things back up enough to allow you to advance the timing back to stock. I would also hit the carb with some carb cleaner, and check PCV valve. PCV is a mixed blessing as it catches the blow by, but reintroduces the oily blowby into the intake manifold. I good carb spray down should help remove some of the blow by gunk.