yep sounds like your secondary side of your carb is a bit crook it will be opening at 4100-4200rpm and causing the sound change in your motor, it sounds like your problem is intermittent but the easiest way to try and fix it would be with some fuel system cleaner and a good going over the carb with some nulon carby cleaner to try and disolve any deposits and help restore normal opperation.
Failing that it could be time for a carby overhaul, about $60-70 for a full kit or maybe get a hold of a weber 32/36 DGV carb and adapter plate if you want a bit extra power