1971-1973 Arctic Cat Snowmobiles Factory Service Manual, Page 77Get this manual
CARBURETION (NALBRO) General
Walbro carburetor models WD and WR incorporate straight-through primary venturi eliminating the restriction of secondaryA large diaphragm assures precise fuel metering athe built-in fuel pump delivers the necessary fuel, thus eliminating "flat spots"The fuel is filtered internally and vapor released by unique pumping action preventing vapor lock due to high temperature or high altitudeVENTURI venturi is constricted area where air is allowed to pass throughBecause of the venturi design, pressure varies between the inside of the carburetor throat and the outside airThis is to allow better atomization of the fuel in the carburetor resulting in better engine performance at all operating speedsWhen air moves through the venturi, the velocity of air flow is increased due to the restriction of the venturi areaNOTE: The venturi also produces vacuum at the point of maximum restrictionThe Walbro Carburetor utilizes power needle at the maximum point of restriction, resulting in mixture of fuel that has been drawn with incoming airThis process of mixing fuel and air is known as vaporization