I measured the orifice at about 2.95 mm and I measured 120 mL/min of fuel through one bowl with float and needle in situ but tank was only 1/2 full. WTH, I may just send you a spare to examine.
Some Amal problems were due to the flexible plastic of the float that flopped around (fixed by using a StayUp float or an old hard float) and some were due to the float pivot pin being loose and held in place by just the bowl gasket (fixed by lightly tapping or staking next to the pin). Float height varied a lot from one carb to the next direct from the factory, so each carb should have the height set properly (heat the bowl and tap the brass seat). The Concentric was introduced as less expensive to manufacture than the Monobloc and the result was rapid slide wear, erratic idle, no replaceable idle jet, etc. Looking at your website, it looks like I am unlikely to make a convert out of you. A local group is taking a blown Honda to Bonneville and they can't get their Mikuni set up right. I offered them an Amal, but they don't believe I could set one up right for them, either.
Anyhow, carbs are carbs.