There's some thing that can cause this issue:
1. The points cam may be worn on its inner pivot. This lets it swing (or wiggle) "sideways", so to speak, when the weights reach full advance, which ends up adding extra advance.
2. You might be using Daiichi brand points. These monsters cause all sorts of these timing problems because their geometry is wrong between the rubbing foot and the pivot. This forces you to have gaps outside of the range of .012" to .016" to make them reach the static (idle) timing marks, and when the advancer swings to full angle, the points then double-bounce on closing (especially if the gaps are at .016" or more): this makes it look (to a timing light) like it is advanced way past the advance marks (I know it sounds confusing, but that's what it is doing...). If you happen to also have one of my Transistor Ignition boxes, the spark comes on again, full-force here, and makes the engine backfire with a 'sputta-sputta' sort of sound out the pipes. It's because the points are giving a false trigger signal at the wrong time.
3. The springs on your spark advancer are likely annealed and have become too soft now. This is a common malady, probably because Honda never intended these bikes to be around 40 years later(!). On many of them, like the 750 and 450 twins, cutting off 1/2 turn from one or both springs is a good way to let them settle back to idle - especially when hot - and gives them a much smoother, more controllable, launch. This 'fix' stops their sudden urge to jump high in RPM when the throttle is first cracked open, and only delays the full advance by about 200 RPM, so you'll never notice.
