Replacement Rotor 61HB GGT10 for O.S. Carburettors - O.S. Engine OS4A408120  [OS4A408120]

What the rotor does

The O.S. Engine OS4A408120 rotor, stamped 61HB GGT10, is an aftermarket replacement rotor for matched O.S. carburettor housings on RC nitro engines and is used to restore proper metering geometry.

Functionally the rotor governs the air/fuel mixture through the throat, shaping idle and throttle response. Always confirm the rotor code and indexing match the carburettor slot to ensure correct fit and orientation.

Installation notes: remove the carburettor top and withdraw the old rotor while noting its indexing, inspect the slide and needle for wear, then insert the OS4A408120 with its marks aligned to the housing. After reassembly, run bench idle and midrange tests and adjust the needle gradually to fine tune the mixture.

As a tuning element the rotor can change throttle progression; fitting the correct profile often returns consistent throttle behavior. Keep sealing surfaces clean and replace any worn gaskets to avoid air leaks that interfere with tuning.