I agree with bkap, it sounds to me like it is going dead lean. You said you rebuilt the carb, so that is where I would start at this point. Is the float set too low? Needle and seat stuck partially closed? As stated before, it is hard to diagnose from a distance, but it certainly sounds like it is going dead lean too me. Mufflers or the lack thereof will have absolutely no effect on the timing. They will effect the combustion in the cylinder. With headers and no mufflers, the engine should run leaner than with the mufflers installed. No mufflers should make the problem worse if it is a mixture problem. If I remember correctly though you stated that it ran better with no mufflers..........don't know.