not sure if the SG of modern fuels is different from those back in the day.
Well, since you asked...
Making a long story short, almost 20 years ago I did a series of experiments to help a friend get his jetting close if he raced at the bottom of Death Valley one weekend in summer, and at the top of Pikes Peak another weekend in winter. This would let him quickly zero in on the right jetting and spend more time getting to know the course.
Anyway, Amal's jet testing specifications call for fuel with density 0.710 g/cm3 at 15 ℃ (59 ℉). I measured the following densities for four brands of gasoline that I purchased from busy stations (i.e. with a high turnover of fuel) in the summer.
Chevron __ 0.710 ___ 90 ℉
Union 76 _ 0.740 ___ 87 ℉
Texaco ___ 0.749 ___ 88 ℉
Exxon ____ 0.758 ___ 88 ℉
Without going into the (interesting) details or consequences, note that all these measurements were made at temperatures ~30 ℉ higher than Amal specified, so all would have higher densities if measured at 59 ℉, but even at elevated temperatures they have greater densities than the fuel Amal specified back in the day.
The flow rate through a jet depends on the density/viscosity (plus other factors) and, like the density, the viscosity also increases as the temperature is lowered, although with a different slope. I also measured the density and viscosity as a function of temperature of the brand of race gas he used at that time (VP C12), since using it eliminated the variability of "ordinary" gasoline. I programmed all of this into a calculator so if my friend had found his jetting to be perfect at, say, a race in Death Valley when the temperature was 120 ℉, plugging that jet size into the calculator along with the measured temperature and RAD at a different track would give him the new jet size to use, which would be very close to perfect. At least, down to 25 ℉. If he had been racing snowmobiles, I would have extended my measurements to lower temperatures...
Coming back to what Greg wrote, what the above means is if you built a jet-testing rig like Amal specified and used, say, Exxon fuel in it at 59 ℉, the flow you would measure in cc/minute of a, say, 200 Amal jet would not be 200. My guess is Mikuni used the wrong fuel when they set up their calibration rig decades ago, which is why their jets flow 16% different than Amal jets.