I have been a firm proponent of using Seafoam in my gas tank. Recently (like, day before yesterday) I noticed the engine bogging down whenever I climb past 9K RPM, and it doesn't matter which gear I am in. Prior to that I've had very few issues with the carbs running lean. My best guess is probably clogged mains.
I welcome alternate viewpoints though.
Just to clarify further, it is more of the engine cutting out like you feel from a limiter then slowly slowing down.