I had a chance to take the bike for a spin or two around the block, and it runs a lot better over 3k rpms, because of the absent air-fuel screw for the #4 carb. When the screw is taken out, that cylinder gets an overdose of fuel at idle. It's misfiring. At higher speed, the main jets are dominant, and it's less of a problem.
I can put a screw in, but it only stays in when it's in all the way, which causes it to run too lean (i.e., zero) at idle. I need to find a way to repair the threads, or I'll have to toss the unit.
On my test run, I used a makeshift seat fabricated from a piece of quarter-inch MDF, two folded up bath towels and a plastic bag. It actually stayed on quite firmly (because of the battery box screws holding it in place), and was surprisingly comfortable, as it molded itself to my posterior. I'm going to make a real seat, and use this one as a prototype.
It was funny -- I didn't rev it too much in case the oil wasn't circulating properly. But as I eased the bike up into second gear, the power of the acceleration almost left me lying in the street. I had to grip the tank with my knees just to stay on. It may be a 78, but it has some serious pull.