Biggest 389 is 1 3/16 which is about 30 mm. My A10 has a RR or SR head (some questions remain over the parents
) and has a 930 Concentric. I'd quite like a 389 as they are a good carb.
But the inlet on the head was about 1 1/16 to 1 3/32 and it came with a 626 Concentric which made no sense. I opened the inlet up to 30 mm to match the bigger carb and got a matching tufnol spacer.
Bear in mind the late A65 had two 30 mm carbs and they start and idle perfectly well.