I've got a 1984 Honda Elite 125 scooter and it misfires at top speed. At WOT, it pulls up to 48 MPH and starts popping and surging. Backing down to about 90% throttle smooths it out and brings it up to 50-51 MPH. I've tried changing the main jets and it makes no difference. It changes how hard and smoothly it accelerates, but it still starts stumbling at the same point.
I was thinking it had a rev limiter of some kind, but changing the CDI also had no impact.
So far, the carb has been replaced (PO put a 250 carb on it for some reason and it still had the same issue, but was running FAR too rich). Ignition coil, spark plug, plug wire, and CDI have all been replaced as well without change.