The standard Speed Four bar end weights are rather heavy. I think your problem is caused by gripping the throttle too hard. Try to soften your grip a bit.
You might also wanna try a throttle rocker
Interestingly, btw, when I once had to ride the Speed Four with standard clip ons without bar end weights, the vibrations drove me up the wall. Now that I have LSL clip ons without weights, it doesn't seem to vibrate at all!
