As you no doubt know, the bars are where much of any vibrations from anywhere on the bike are felt through. So if the bar end weights are screwed on nice and tight, you've gotta start looking elsewhere. But there are many things that can cause vibration thro' the bars - tyre wear, carbs out of sync, chain rattling - the list goes on. Everything else seem Ok? Is the bike generally well looked after? Can you hear anything that might give you a clue?