With steel balls it will work, but it can be time consuming. I have my own method of bench syncing. I use a small allen key under the slide engine side.
Here's an idea anyone of you could try to see if it works. I haven't tried it myself yet, because I'm too lazy to remove the rack and put it on the kitchen table just for that.
The basic idea is to visually 'enlarge' the differences in gaps appearing at the bottom of the slides engine side. This by using the principle of leverage.
You'd need four identical say allen keys or sticks and position them in an identical way, one under each slide, protruding forward. You may need to turn the idle knob for this, but that's no problem; after finishing the syncing, you can readjust the knob ofcourse for the right idle.
Position the rack on the table like it is on the bike.
Insert 4 small allen keys in the gap under the slides. I don't remember if there's any slope in front of the slides, but the idea is to turn the idle knob thus that the allen keys (or sticks) are all free from the slope in front of the slides. Depending on the length of your four indicating sticks, even a small difference in the slide opening will be visually translated in an 'exaggerated' way so to speak, by the ends of the sticks (because of the leverage). Now adjust each carb sync screw thus that all four sticks 'dangle' at the same angle, which will be easy to see, because of the leverage which exxagerates things. I hope I've made myself clear. Please don't hesistate to correct me.
For those of you, who have the carbs completely apart, a situation like in the pic below, I can indicate how to come to the right basic setting, the way Keihin must have done it. However I'm a bit reluctant to publish it, because, frankly, the process of dissasembling your carbs to such a degree, is never needed. Gruzzel in Germany has done that anyway, has made the basic setiing and did not even need any further dynamically syncing afterwards.
