That could easily be the problem. As I mentioned in another thread, many motorcycle carburettors use the idle and transition ports as a power enrichment circuit when the throttle is open wide and the engine is under a load. In the range above peak torque and near peak power, the mode of the intake system gets really interesting. The valve timing and port configuration start governing the air flow in that region, so the manifold vacuum starts dropping. There is a huge amount of air flowing through the carburettor(s) but not a lot of pressure difference from inlet to outlet. Since the speed of the air flowing over the ports is high enough able to pull fuel out of them as easily as the high vacuum at idle, you get the richening effect you need, unless the idle mixture is off.
On my America, when I took it in for the 500 mile service the wrench tuned for minimum exhaust popping. After the 1K mile runin was done, I followed the books advice to occasionally "ease" a new engine up to the rev limit, only it was in top gear, which I don't think they intended. I could barely top 100 on the speedometer. After readjusting the idle mix for fastest idle speed, I tried again in the same place under similar conditions and backed off after reaching 115.
