Hi Doug,
The 90 Plus was supplied with various specifications dependant on the customer's intended use. The 'standard' machine equipped for road use had Amal 276 clip fitting carbs (RH 276EP/1B, LH 276EQ/1B) of 15/16" bore with a 6/3 slide, No.6 needle and a 130 main jet (for standard compression ratio). The list of extras for converting to a racer included TT type carburettors. With more tuning, some of the later models were running 150 main jets with silencers fitted, and 170 mains when running on an open pipe. Unfortunately, my information sheets don't give any dimensions for port or valve diameter.
Hope some of this helps.
Regards,
Eddie.