[Synchro909]

Remove the carburettor and back the idle stop screw right off till it is clear of its stop. At this point, it is the plate that is stopping it closing any further. Then loosen the two screws holding the throttle plate in the shaft. While holding pressure on the throttle arm in the close direction, tap the plate lightly with the handle of a screwdriver. If the plate is not quite right, this will move it to the correct position. This will make sure that the throttle is closing all the way. You will know when it is is right because the tiny hole in the throat of the carby at the float bowl side will be half open, half covered by the plate. Retighten the screws, reinstall and reset the idle speed. Likely, it will be good. All that said, if the shaft is loose in the body, all bets are off.