It happened to me, too, twice. It was the crankshaft right Woodruff key both the times. It could also be one of the two counter-shaft Woodruff keys, but it's less likely.
The problem was that the crankshaft key broke and so the two gears hold in position by it didn't rotate integral with the shaft any more. One of the gears drives the clutch, the other drives the counter-shaft gear. The latter must be in phase coincidence with the countershaft gear; when the key brakes, it isn't any more and so the bike shakes much more.
Here is the procedure:
-Remove the right cranckcase cover.
-unscrew the cranckshaft nut after having bent its security washer; you'll need a 38 mm wrench, if I remember well:
Now, without the friction of the nut, you should be able to understand whether the key is broken: if it is, the two gears will be able to rotate easily around the shaft.
In the picture above, the crankshaft is damaged because I didn't align the crankshaft and the oil pump shaft:
don't do the same mistake!!
The oil pump shaft is here:
-Remove the clutch.
To do it, first of all you have to unscrew these six bolts and to remove their springs:
Then, you have to remove the plate and other little parts you'll find under it (don't lose the little ball).
Now, bend the security washer of the clutch and unscrew the central bolt (27 mm wrench, if I remember well); don't mind the red arrow, I re-used the image:
-After having removed the clutch, you'll be able to remove the two gears from the cranckshaft and check the Woodruff key.
Here are the Woodruff key codes:
-right key on the crankshaft: 8000 22426
-Counter-shaft Woodruff Keys: 1513 71301, less likely to be broken.
When you re-assemble everything, align these two dots to get the phase coincidence I've mentioned:
I hope not to have forgotten anything.
