Honda-SOHC

When I test drove this bike, it seemed to run fine. It started right up (from dead cold, I checked to make sure the seller didn't pre-warm it to make it start easier), and ran great on the little test drive I took around the neighborhood. I read up on the model, liked it, and bought it.
It did fine running around town at moderate speeds, I gave everything a thorough looking over and decided it was safe to take out on the highway to see what she could do. Aw crap. Once it got up into the upper third of the rpm range, the engine seemed to bog down, loose power. Plenty of power at low revs (seemingly anyway, I don't have a basis for comparison, never having ridden a CB750 before).
Then it became harder and harder to start until she wouldn't start at all. I checked for fuel flow, seems fine on all four carbs when I unscrew the drains on the bottom (forgive me for any poor jargon, I am a bit green). I installed new plugs, checked for spark, and the plugs I checked seemed weak