amal pilot jet circuit.

A quick answer, NO! The mixture screw and the idle speed screw work together to achieve the best slow running. The air the engine ingests comes via the slide. The fuel for combustion comes via the small drilling in the idle mixture circuit and is picked up by the air coming from under the slide. If you drop the slide right down, there will be no air to pick up the fuel from the drilling just past the slide.

If you cant achieve a good steadyidle, you may have a worn slide in the carburettor, amongst other things, like worn valve guides, ignition timing,etc.

If you can't slow it down then air (and fuel) is getting in ,worn slide ,bowed mounting or cable with no slack. You may have additional issues ,bad advance unit , try coating the slide with grease which will reduce the air leaks around it and give some idea ,may also make a weak mixture into a rich one. fuel level may be high ,I have 20 thou removed from my float needle housing to lower the level.

Can I assume this is a concentric carb. It may also be applicable to monoblocks.

This may be a classic case of not seeing the wood for the trees. I had similar goings on with two Commandos in the past. Take out the slide and have a GOOD LOOK at the slide. They sometimes break off at the bottom square edge at the rear of the slide therefore letting in more mixture, hence high revs.

The following documents may be useful:

http://www.jonsnell.co.uk/images/tuning_and_using_amal_mk1_concentric_carburetters_.pdf

https://www.princeton.edu/ssp/65-cub-data/library/amalbritbike.pdf

There's other interesting material at the Princeton site.

If the throttle valve is fully lowered you won't get any flow over the cutaway side of the slide because there is no gap at the other (engine) side of the slide to allow a flow. You probably need about 20thou of gap under the engine side of the slide as a start point. This will allow a flow, create a depression, and draw fuel through the pilot system.