I followed Bandid's advice and went to Radio Shack. The washers were only 2 cents each, but I had to buy a bag of 100. Five different diameters, of incredibly small thickness, so one had to fit. I propped up the gas tank and took off the top of one carb. I could see trouble.
The piston is plastic. Looking down inside the piston, the nut holding the retaining plate is plastic. Slipped a 7mm socket on the nut and stripped the head. My speciality is stripping phillips head screws and breaking bolts, but this is the first time I stripped a nut. Tried a 1/4, but it was too small. Tried a 9/32 (How many have 9/32 socket?), but it also stripped the nut. Frustrated, I put it back together.
I checked the little rubber caps that cover the carb-synchronizing vacuum tubes, hoping one was missing. A missing cap would make the bike run lean. Nope, they were all there. I hadn't synchronized the carbs when I cleaned the pistons, so I got out the old Carb Stix.
The synch was way off! No. 1 cylinder was two feet up the stick, No. 2 cylinder was about 18 inches up the stick, No. 3 cylinder was about a foot up the stick, and No. 4 cylinder was not moving the mercury. I first thought that No. 4 cylinder had a hole in the piston, but as I adjusted the screw for No. 4 it came back up. Here's how it looked when I finished. And it ran great after the synch.