First off - that thing is running horribly. Possibly on 2 cylinders.
1) You won't have any luck tuning the carbs without everything else being right.
- So check the valve clearances. And check for flat lobes while you're doing it - anything flat enough to cause any issues, especially at idle, will be VERY obvious to the eye, no need to actually measure valve lift, just look for a glaring lack of lift.
- Check the plugs and wires and cap
- Check the timing
- Check the compression
2) If it still runs like crapola, shut the motor off and pull off the float bowl covers and see if
- there is gas in both bowls
-roughly the same amount of gas (fill level) - there is a procedure to set the proper fill level by bending a tab on the float arm/lever.
-check for crap in the bottom of the bowls - sludge, rust, water, crap, whatever. It's supposed to be filtered before it gets to the carb, small bits of stuff getting to them can end up blocking the inlet to the hose that lead to the jet.
Good news is you don't have gas pouring out of the overflow vents on the carbs. *golf clap*
I've run B18 carbs on a B20 before without rejetting them, it certainly doesn't cause gross running issues like that. There's something else wrong here.
PS: When you lift the pistons to check mixture, how are you doing it? Via the built in pins, or just pulling the pistons up? How much are you moving the pistons up?
With the car off (because there's no point in doing it with the car running) pull both pistons all the way up and then let them fall freely. Do they descend smoothly and somewhat slowly? Do they both go down at the same speed? What sort of oil is in the dampers? Any signs of the needles even faintly touching the jets at any point? Do the pistons both go ALL the way down? Unfortunately, despite being built in an age of interchangeable parts, the pistons and domes on SU carbs aren't very interchangeable. They're usually hand fitted in the SU factory for a very close fit - without any sort of seal between them. Mixing and matching them later on doesn't always work well. And the jets need to be centered on the needle, and the needles can't be bent at all.
There are also some BAD tuning recommendations out and around on the internet which say to lift the piston on carb A and then set carb B based on the results. And some people who will vigorously defend the technique. It's just plain wrong (ask them how they'd tune a Mini with a single SU carb, or a jaguar with 3 SU's, if you are supposed to adjust the 'other' carb? lol).