O.S. Engine OS4A408120 Carburettor Rotor 61HB GGT10 for O.S. carburettors  [OS4A408120]

Carburettor rotor overview

The O.S. Engine OS4A408120 is a carburettor rotor identified as 61HB GGT10 intended as a direct replacement rotor for compatible O.S. carburettor assemblies on RC nitro engines. It is an RC-scale accessory designed to restore or maintain the rotor profile inside the carburettor body, and it is described here in neutral technical terms for hobbyists focused on maintenance and tuning.

Functionally, the rotor controls the metering of air and fuel through the carburettor throat as part of throttle response and idle adjustment. When assessing fitment, verify that the rotor code printed on the part matches the rotor slot and indexing features in your carburettor body, and that there are no burrs or deformation before installation.

Installation notes: remove the carburettor top and slide the old rotor out while noting the orientation, inspect the needle and slide for wear, then fit the OS4A408120 rotor so the indexing marks align with the carburettor housing. After reassembly, perform basic idle and midrange checks on the bench before running the engine, and make only small adjustments to the needle to re-establish proper mixture and throttle response.

The rotor plays a tuning role by affecting how quickly the engine responds across throttle positions; swapping rotor profiles or restoring a worn rotor can help regain predictable throttle feel. Keep all parts clean and use correct gasket seating to avoid air leaks that would mask rotor-related symptoms.