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Carburettors can be very polluted. Most people know that and will clean the carburettor carefully.
However, often they forget to check the little hole of the needle jet. This hole provides the mixture with some extra petrol when the throttle is nearly closed.
When your engine has stationary problems this may be the cause. For the 12 mm carburettors and higher you must dismantle the needle jet at the underside of the carburettor. For older models (without needle jet holder) the needle jet must be removed and inserted by way of the throttle valve bore.

Some examples of needle jets with holes for stationary speed.



Bron: http://www.pantin.nl