Primitive carburetor controls

  Before "sophisticated" carburetors that mixed fuel and air in proper proportions were developed, it was the job of the rider to keep the balance. In the early part of the last century, that was done using separate levers on the right handlebar for fuel and air.  Probably a point of honour for an experienced rider, measured in fuel mileage and lack of puffs of black smoke but remember, the riders hands were also controlling spark advance as well as the usual clutch (if used) and brake levers.