I was doing a track day at Gingerman over the weekend, when the motor started making a sound like a shaft was loose. It almost sounded like it could be the conterbalance shaft. Motor didn't seize or lose power. Didn't feel like the tranny, as the bike didn't start lurching.
Just had what sounds like the same problem on my last track outing. The engine was running fine, but there was a horrible rattle from the bottom end at higher rpms, and it started to vibrate a lot.
I tore that engine down, and the balance shaft has about 2 or 3 mm of play at the ends. I don't think that is how it supposed to be! Lots of darkish metal "dust in the oil also, which I guess is the remains of the balance shaft bearings.
Very common failure to have the balance shaft bearing go during some high speed use. Poor little air cooled thing! Gets too hot and oil starved ! See the bit in "Racers Corner".
So...........how hard to replace? What if I just took it out (and plugged up the oil journals)? How bad would it vibrate?
There has been threads about removing the countershaft - it has been done. I guess it vibrates quite bad. It can be made somewhat acceptable for racing by having the crankshaft rebalanced, but this is not always very successful.
Bob Broussard or Bob Sunday can tell you a lot more than me.
Replacing the bearing, or taking the shaft out requires splitting the cases. It's not that bad... I'm just starting a ground up re-assemble job, with the balance shaft.