Wait I went back and looked there are oil lines there so now I am guessing some kinf od head oiling system. Maybe they can't get enough oil to the head during the runs. It is not a supercharger since that would need to be connected to the carbs and head.
"Remember that an enduro tests the endurance of three things: your machine, your body, and your wits. Only one has to fail to keep you from reaching the finish line." Cycle World March 1966