I've seen glowing headers, and guys thinking they were running lean, so they added fule. Big mistake! Both guys are corect, above, although I'd be more wary of the retarded timing than a too-rich condition. Retarded timing can burn ex valves in a hurry. A combination of both is asking for trouble, and since you've admitted playing with both the timing and jetting, my guess is that's what's going on here.