Early motorcycle throttle control

 

Early carburetors were less than efficient and required constant adjustment for both fuel and air flow. One solution to controlling the mix was with two levers on the right handlebar. The long lever controls fuel and the short one, air.  The above sketch shows positions of the two levers in controlling fuel and air to the carburetor in different conditions. Imagine trying to modulate while starting without a clutch and almost non existent brakes. 

Below, exploded view of the Brown and Barlow twin lever control.