Doesn't sound like a fuel volume issue. On the street at cruising speed the 3/8" line will supply more then enough fuel. At first I thought we were talking some end of the quarter at 7300 RPM deal. Does the sputtering occur at a specific RPM every time? Sounds more like a timing issue, maybe the centrifugal advance is floating around a bit when it pulls, coming in to slow maybe? All the advance should be in by 1800 to 2000 RPM, though. Maybe a weak plug wire (can check them with an ohm meter), or a plug or two breaking down a bit under load....