Sluggish acceleration above 5000 RPM

[Ray]

(Or a harmonic.... Not sure you can pick up the sensation of a 4 kilohertz vibration through your feet. The changes in acceleration suggest the problem lives downstream from the engine.)

4 to 5000 rpm is 67 to 83 hz

Yikes! I was asleep for that one.... Seconds vs. min.... Geez. 

And (correct me if I'm wrong) only half that if you are only including combustions in a four-stroke with two cylinders(?)

(You know, I recall a moment of hesitation while adding a comment on physics or engineering to one of your posts....)