On the surface, it sounds like a very lean spot in the air/fuel curve.
Happened to me after I bought my RC51.
The prior owner must have never gotten the exhaust mapped.
On mine, at about 5800 RPM, the power began fading away, and at 6000 RPM, it was so lean it started to misfire.
Past 6200 it picked up and ran fine.
A forum community dedicated to Honda RC51 owners and enthusiasts. Come join the discussion about reviews, performance, V-Twin racing, modifications, classifieds, troubleshooting, maintenance, and more!