Enjoying your thread.
Both my 2-speeders were former carburettor spitters…
Neither have an inlet splitter or gauzes fitted, one is running on a 1” Amal 276 with a 190 main jet and a 3 1/2 slide and the air screw out 1 turn, the other is on a 1 1/16 Amal 276 with a No. 3 slide, the air screw 1/2 turn out and a 210 main jet…
In my case I found several conditions to increase the likelihood of spitting back through the carb:
1) Retarded ignition
2) Air screw too far out (too lean)
3) Cutaway too lean or slide / carb body worn
4) Air leaks
5) Running out of fuel
I would be tempted to try a richer cutaway on the slide, turn the air screw in a touch, double-check for any air leaks with soap suds on a running engine and check the timing…
Exhaust spitting I have no experience of, so forgive me if this is all superfluous.
A study on port sizes appears in technicalities as I recall either Potty or Tim Sharp measured a multitude of barrels and shared their findings