Severe vibrations

Some frames and some kits seem to vibrate more then others. Sometimes with time it gets better.

http://motorbicycling.com/f30/motorized-bicycle-vibration-1024.html

http://motorbicycling.com/f3/bicycle-motor-vibration-511.html

Also balancing your wheels might help

http://motorbicycling.com/f30/motorized-bicycle-wheel-balancing-310.html

http://motorbicycling.com/f13/motor-bicycle-wheel-balancing-188.html

They smooth out with miles. A few hundred should do it.

On my particular build, the greatest vibration came from the chain. If you were to take the slack out of your chain (best to remove it altogether) and you were to take the individual links and squeeze them to go through their full range of motion...something like this: __^__
If you can't freely fold the links, voila! Just maintain the shape indicated above and give a good hard side-to-side force...to exert the sideways force to the links that are stuck..should free them up nicely.

I agree- most of the vibration from the bikes I've built (after break in) comes from the chain.

Just don't make it too tight or your tensioner will make severe vibrations on your spokes.

3/4" will do it.

Out of curiousity, to verify the chain vibration, get to good speed and kill the engine/pull in the clutch. It should be smooth and quiet. If not...chain needs fiddling with.