Originally Posted by Wamsutta
Not a one?
I'd say its unlikely that the wheel suddenly went that far out of balance unless you'd had a huge balance weight fall off or something. So here's what I'd check.
First check for a tire that's starting to come apart meaning the cord has torn inside causing a deformation in the tire basically spin the wheels and see if the tire spins true or has a "bump" anywhere on its surface. Spinning the wheel will also confirm that a rim hasn't gone out of true due to impact damage.
Next check the wheel bearings turn the wheel slowly by hand and make sure they rotate smoothly then hold the front and back of the tire and try to rock the wheel side to side to make sure there is no play in the bearings. Assuming the wheels are ok next check the chain and sprockets a severe tight or loose spot in the chain can show up as a high speed vibration as can a countershaft bearing thats starting to come apart. To check put the bike on the centre stand or support it some other way with the rear wheel off the ground if you don't have a centre stand. Next remove the little cover over the front sprocket then spin the rear wheel and watch the chain as it rotates around the sprockets does it have significant kinks or severe tight or loose spots. Does the front sprocket (or rear for that matter) rotate smoothly, even if it does use a screwdriver to lever gently against the sprocket to make sure there is no play in the bearing that supports the shaft the sprocket is mounted on.
Those are the obvious things I can think of on the chassis side and they are easy to check so maybe have a look at those first. If they are ok then the problem may be related to the engine and further investigation will be required.