The amal 289 doesn't have a pilot jet but mixture is controlled by the airscrew alone. If the airscrew needs to be almost fully screwed in for decent idling, can there be a partial blockage in the pilot circuit, and if so how can it be cleaned? Squirting carb cleaner through the screw hole has not had any impact.
Stick it in an ultrasonic cleaner.
Or it could be that too much air getting into the carb on tickover which could be a leaking manifold or a worn throttle slide?
If I can pass this post on to my preignition topic as the two are closely related.
I removed the jet block to thoroughly clean the pilot system. It is still running weak with the airscrew fully home. I am pretty sure that the manifold is airtight and the throttle slide is a good fit having been replaced in the not too distant past.