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

Rotor function and fit

The O.S. Engine OS4A408120 rotor, marked 61HB GGT10, is a direct-replacement rotor intended for compatible O.S. carburettor assemblies on RC nitro engines and restores the correct rotor profile for reliable metering.

The rotor meters air and fuel through the carburettor throat, directly affecting idle stability and throttle response; verify the rotor code and slot indexing to confirm correct orientation and fit prior to installation.

Installation notes: remove the carburettor top and note the orientation of the old rotor, inspect the needle and slide for wear, then fit the OS4A408120 so the indexing marks align with the carburettor housing. After reassembly perform bench idle and midrange checks and make only small needle adjustments to re-establish the proper mixture and throttle feel.

The rotor influences how the engine responds across throttle positions; replacing a worn rotor or restoring the correct profile can recover predictable throttle progression. Keep components clean and ensure gaskets seat properly to avoid air leaks that can disguise rotor-related symptoms.