Yammies do have a limiter on them. Limiter is on at 6350-ish, off at 6200.
Almost sounds like the limiter's kicking on. My 4 stroke does exactly that with the stock 10.5 prop. Almost sounds like kind of a faint lawn mower surge at top speed. Put an 11 1/4" Yamaha 2 stroke stocker on, picks up lots of MPH, and RPM is down to 5900..right where it should be.
Could be carbs too...but usually a lean carb will show up long before "top end". Bog usually. A rich carb/carbs can sometimes surge too, but "usually" that'll show early as a rough running condition between about 3000-4000 RPM just as you're getting on plane.
If it's rich....I'd question how the jets were cleaned. SOme guys like to use welding tip cleaners or drill bits or steel wire, all of which enlarge the orifice a little. If it's lean, I'd question whether they were properly cleaned or not. It's REAL easy to get sidetracked and/or complacent when doing these carbs, especially since the techs can and do have to clean a ton of carbs nowadays. Ya gotta know what you're doing with these...not everyone knows how to do them properly which includes double checking timing and synchronizing them after removal/splitting/disassembly.
