Filter body. Models: FS 45 FS 38 FS 38 2-MIX FS 55 R FS 55 RC-E FS 55 R 2-MIX KM 55 R MM 55 The transition from the carburetor with LD screw - BIS, Balanced Idle System - (on the left) to the carburetor with H, L and LA screws (on the right) involves the use of the air filter cover on the right, replacing the one on the left.