OH This is the best one to cover.
The best car technicians and inexperienced carburetor tinkerers have used this verbiage to explain what they could not find or explain.
To set the record straight…CARBURETORS DO NOT HAVE INTERNAL VACUUM LEAKS. There are vacuum leaks from intake manifolds, egr valves, broken vacuum lines, distributor vacuum advance diaphragms etc. Carburetors can have restricted idle transition or main circuits. That is NOT a vacuum leak. Any of these may have the same symptom as a vacuum leak but does not mean there is one, especially from the carburetor. Just because you have a headache does not mean there is a tumor in your head…does it?
This brings us to the technician spraying carb cleaner at the throttle shaft of a carburetor on a lean running vehicle. All he has done is confirm there is a lean condition.NOT the cause of the lean condition.
All engines require certain amounts of mixture at idle. When that is disrupted you will have a lean or rich mixture. If the intake manifold is good and all vacuum lines have been checked for breaks and cracks, egr valve has been tested then you investigate the see if your carburetor has a restricted idle fuel circuit. That is the ONLY way your carburetor can give you a lean condition.