For the past two years I have been riding a 2004 300 EXC that I have completely rebuilt to factory new spec and better (even had US chrome redo the cylinder!) and a Keihin PWK38 carb with a JD jet kit. It has always been "busy" at speed, with current gearing above 65mph there is always some pipe bang/surging/bucking at partial throttle (give me full power or nothing) with a thin margin of happy throttle zone to maintain speed that gets thinner the faster you go. I have assumed this to just be the nature of the engine and accepted it, as it behaves like most other (smaller) aggressive two strokes I have worked with. Power valve tuning did help this, opening later to keep it off the pipe seemed to make it more well behaved at cruising speed. Everywhere else it is fantastic, excellent power and response, just doesn't like to sit still. The usual fuel is REC90 with 60:1 Motul 800, on a rare occasion Klotz super techniplate when I feel like smelling nice. This past weekend when I was at the track spectating at an FMRRA race I decided to put some race gas in it (because why not). I put around 2.5 gallons of Sunoco Maximal 116 mixed 60:1 with Motul 800 in the tank. Aside from the awesome smell, the engine dynamics have totally changed. I can now cruise happy at any speed without any of the aforementioned ill-behaviors and it seems all around more happy and responsive (definitely feels like it's making more power at WFO, but I have not dyno tested it yet). I am totally stumped as to why this made such a huge difference under vacuum, this fuel is not oxygenated, so I can rule that out. Night and day difference, anyone have any insight as to what is going on here?