gate valve near the carburetor that admits warm air from a scoop on the exhaust system. The R22 employs a carburetor air temperature gauge, marked to indicate temperatures conducive to icing. The Beta II version of the R22 also includes a "carburetor heat assist", which automatically applies carburetor heat when the collective lever is lowered below a certain point. When icing conditions are present, carburetor heat is required to prevent icing around the butterfly valve from the pressure drop at that point. As the carburetor air temperature (CAT) indicator does not read correctly below 18 in Hg (457 mm Hg) of intake manifold air pressure, icing conditions require applying full carburetor heat below 18 in Hg of manifold pressure. A placard indicating this requirement is located on the CAT indicator and in the pilot's operating handbook. Power is transmitted from the engine to the drive system through drive belts. Originally, the R22 used four separate v-belts running on multigroove sheaves. This system proved problematic, as individual belts sometimes rolled over in their groove and fail. As a temporary measure, in 1982, R22 operators received a kit from Robinson that was installed in the cockpit and on the belt tension actuator, isolating the tensioning circuits and locking the clutch/drive system at take-off tension. The problem was ultimately solved by replacing the four individual v-belts with two dual v-belts. The upper, driven sheave is mounted on the main/tail rotor drive shaft incorporating flexible couplings, and is raised and lowered relative to the engine-mounted, driving sheave by means of a belt tension actuator. During shutdown, the actuator is used to lower the upper sheave to loosen the drive belts. For startup, the engine is started with the belts loose, allowing the engine to run without spinning the rotor system. Immediately after engine start, the clutch switch located in the cockpit is closed by the pilot, powering the actuator to slowly raise the upper sheave to flight position, which tightens the belts. The actuator is thereafter controlled by pressure-sensing column springs, automatically maintaining proper belt tension during flight as the belts wear and stretch. The shaft on which the upper sheave is mounted drives both the main and tail rotors; the main gear box delivers power to the main rotor shaft through a set of splash-lubricated spiral bevel gears. A one-way sprag clutch is built into the center of the upper sheave to allow