Yeah, as posted above, Get a PERRY ! I am ST person and fiddle-faddled with their carbs for years. I must admit, that I was successful with their .61 ST carb and the engine runs smooth with good transition.
Right now I am running a .90 ST (Italian if that makes a diff.) with a Perry. Bolted it on and 10 minutes later after adjusting the idle mixture and idle rpm, VOLA ! That was 4 years ago and I haven't looked back since.
Connelly Engines currently sell Perry with a excellent presentation on buying one. Also, gives specs on various diameters. Go here and spend you money to have fun:
Welcome To the All New Parry Pump Website
RACE 66