My '89 had too much vibration. I had to put buckshot inside the handlebars to make it livable. It's not the first time a manufacturer let part of the bike design be "close enough". If my next KDX is the same way then when I change out crank seals and bearings I will drill 6mm holes in the flywheels to see what happens. If its made worse then I'll have to take it apart and add more metal to that area instead of take it away. I'll post my results.
It vibrated the same after replacing the bearings and although a wobbly crank will cause vibration, I think it is common problem with the KDX since the topic of how to reduce vibration has always been a popular one.
Unless you have a balancer you are taking a crap shoot, the odds of getting it "more balanced" are pretty low. I would get a new fly if you are so sure that is the problem, being a fairly good mechanic, I am sure you already know that as well as the consequences or reducing fly wgt.
Since I started this thread I have devised my own method of crank balancing that I perfected on small engines made for bicycles (since replacement cranks are only $25). You can read about it at www.dragonfly75.com/moto/vibes.html
A good crank balance equalizes the vertical forces with the horizontal so that the pattern of forces throughout the whole crank revolution is close to being a circle when graphed radially. If the counter balance of the crank is lacking then the circle becomes oblong in a vertical pattern. If the counter balance is too much then it becomes oblong in a horizontal pattern. Either too vertical or too horizontal have amplitudes greater than the circular pattern of a balanced engine which produces an annoying vibration at the handlebars. So I am theorizing that the resultant pattern can be seen by using a long thin metal connected to the crank cases while it is running. Here is my proposed method of testing:
Use a short/thin bicycle spoke or section of coat hanger to press onto the engines left side cases so the spoke is sticking out to the left. Start the engine with it in neutral. Press the spoke onto the cases with your left hand and position your body so your eye is inline with the spoke farther to the left and close to it while controlling the throttle with you right hand. Slowly increase RPM as you watch the spoke end. At top RPM it should remain centered. If not then the crank is out of balance. Top RPM is the most important because that is when the forces on the crankshaft are the strongest.
On my Suzuki 100 the spoke end only leaves the centered position around 3300 RPM. Then it vibrates from about 10:30 to 4:30 o’clock (making 12 o'clock the upward direction of the cylinder). Below and above that RPM the spoke has only the slightest of vibration, mostly staying centered. This bike has very little vibration at the handlebars which is one thing I like about it.
If you do this then let me know the outcome. I want to know if this is a good way for a DIY test of cranks out of balance