On an Amal Monobloc or any other carb which also uses a slide throttle, it's cut away primarily to control richness during part throttle acceleration.

These are not constant depression (ie SU or Stromberg CD type) carbs. On an Amal the needle is attached to the throttle slide but has 5 height positions.
Comparison of slide throttle carb components with SU ones is invalid, IMO.
They are different animals in the way they work.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.