With your set-up: Joe's right!
Out of the box, with a rider that weighs around 180lbs...30-32 is absolute tops.
This is assuming the engine will rev to just over 7000rpm. As Joe said, many of them won't.
If you are able to reach 30mph, there's nothing wrong with the engine.
You will need to step down to a 38-40T sprocket if you want to cruise at 30 without the excessive vibration. You can also try to get more rpm out of your engine through modifications, but it will likely get expensive.
If your engine is fairly torquey, get a smaller rear sprocket and your engine rpm will drop out of the vibration zone for the same mph.
...Joe, that is what my dad called a calibrated "whack", and yes it does work!
Vistaman, you could take it to a motorcycle shop that is equipped to rebuild engines. They will have the "V"-blocks and dial indicator to check for trueness after tweaking it.