Does the jet have a groove around it half way in the face or is it a plain face jet?
As below.
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LM i think is the earlier of the two .
vccomm the G4 carby was the earlier model,by 1978 the G4's were replaced by the LM carbys and are still being made today.
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Here is a parts view of how it should be.
The only thing I can think of at the moment is the earlier poppet values use to have letters on them and they had edges on them and for certain mowers they had to be in a certain position.The newer poppet valves however don't.
Does yours have the letters on the inlet side,or is it a plain face one?

Here for a good time,not a long time.