I have owned my 1992 VN750 for three months now, and only been a rider for three months - so I don't have much experience. However, I do know that the bike "feels" different under different circumstances. You mentioned a vibration at 4000 RPM. I have noticed that the smoothness of my bike varies not only with the RPM of the engine, but also with the Dow Industrial average, the price of Pork Bellies, brightness of the Sun, wind direction, relative stability of the Middle East, the Consumer Price Index, rate of inflation, 30 year fixed mortgage rates, and the price of Hot Wheels cars at Target stores. In other words, sometimes it runs great, and sometimes just a little less than great for no apparent reason. I had dinner with some friends the other night, one of whom races motorcycles. He told me the way a bike runs is affected by ambient temperature, atmospheric pressure, and who knows what else. I just try to enjoy the new experience of riding. I do believe the carb sync is a good idea and does affect performance. If you haven't tried it, make small adjustments and see what happens.