base timing keeps advancing by itself...

Using just a plug check for checking rich/lean is fraught with danger. Even with the old leaded fuels, but worse today with unleaded & the variety of blends.
If you suspect the WOT mixture is lean, go 3 sizes bigger on the jets. With such a large increase, it would cure a leanness problem. If it still detonates, then you have other problems. Has fuel delivery been checked, float level etc?