There are 2 things that cause this symptom on CB500, which was highly resistant to this problem by design:
1. Dirty jets or jet holders. One dirty idle jet can do it, or two ro more patrially-clogged emulsifier tubes in the main jet holders.
2. Wrong timing. The most common of this symptom:
a. Points gap is too small or very wide. Too small causes early spark termination, too wide causes weak spark. The symptoms are similar.
b. Timing advance weights frozen on their pivots, or "sticky", causing erratic (or no) change in advance with engine RPM.
c. Electronic ignition with wrong static timing setting.
d. Low battery voltage, aka weak battery.
The bellcrank system on the CB500 has such low gain that you have to grab nearly 2 handfulls of throttle to get it to lean out too much if the tune is right. That's hard to do!
If you have changed the intake air filter system (like, to individual filters on each carb), then you will have to adjust the jet needles to compensate for the sudden change from rich-to-lean at 2/3 throttle. Do this by either raising the needles a notch or by drilling out the emulsifier tubes (see my post about the 750, which goes the opposite direction with the emulsifier problem - but that's a different issue).
