Had the same issues with my CB750 when I brought it home last fall.The acceleration hiccups and backfire was cured by having the carbs cleaned,tuned and synced. The speedo jumped around like crazy then finally quit. When I pulled it off the bike to replace, I noticed the ends of the cable where rounded like a screwdriver bit that has been spun in the screws too many times, the new on had ends that where nice and square. Installed it and have had steady speedometer since.
Don't know if this holds true for the Honda twins, but the 4's steady power band is in the higher revs, like 5k+rpm. Good steady strong pull up there, just breaking too many laws unless on the highway:banghead: