The springs are intact and pull back on the advancer plates when you move them with a screw driver.
As long as you see the spark advance and retard as the revs climb and drop, it's probably OK. A little timing wobble is not unusual in an older 'well experienced' distributor running ignition points.
A balancing tool is a great multiple carb tuning shortcut, absent that you have to do a very iterative process of trying to balance them by ear. Basically, when you screw the idle screw in and out some small amount (like 1/4 turn), on each carb, it has the same effect on idle speed. Screw it in 1/4 turn on the back carb and idle rises some small number of rpm, then go back. Then try it on the front carb and see if it goes up more, same, or less rpm. If it goes up less, then the carb is flowing less air, and needs to be opened up slightly. If it's the same, leave it alone. If it goes up more, close it slightly. And vice versa.
This is (considerably) complicated by the fact that the carbs might not have the same jet tuning - one may be running richer or leaner, thus affecting the idle speed produced from that carb. So after you've gotten them roughly balanced via the idle adjuster screws, then start tuning the mixture on each carb using those lift pins (when lifted, idle goes up and stays up - too rich, goes up then returns - good, goes down - too lean). Then go back and rebalance them via the idle screws, and if it made a difference, go do the mixture again. Until you aren't doing any adjustments on either.
Of course, this is where you have to mention the throttle shaft leaks on a worn old set of SU's. They feature brass shafts rotating in a cast aluminum body with no rubber seals of any sort. When the shaft wears into the softer aluminum (perhaps due to unleaded fuel? Or just... cheap design) it can create an air leak. And at idle, where you adjust these carbs using the lift pins, the vacuum is largest, and the 'metered' air flow is the smallest, so the leaks have the larges effect on mixture, leading you to set them to account for the leaking air, and leaving them too rich at all the other throttle settings.
Back when my PV had SU's it seemed like I needed to do a tuning session on them more or less seasonally. They just tend to drift out of tune for some reason. In small ways.