I've been so busy with school and stuff that it wasn't until yesterday that I finally was able to pull the carburetor from the Wonderbus to do what everyone has urged me to do to solve the problem. Reminder: engine would intermittently and unpredictably die during return to idle. Sometimes it would, sometimes it would not. Ignition tested fine, fuel pressure fine. Engine would always start right back up. Blowing out idle jet did not help.
Pull the carb, they said, check the bowl, blow out what you can. Jan was starting to get cranky that I wasn't getting around to it. I'm slow, okay?
So. Pulled the carb (34 PICT), removed the top, yanked the float and blew out everything that was blowable. There was a dusting of swarf in the bottom of the bowl but otherwise everything looked fine.
Noticed that the automatic cutoff solenoid was loose, not fully-tightened. A bit rattly, in fact. Tightened it. Possible cause of problem?
Re-assembled, re-installed.
ROOKIE NOTE: make certain the float floats easily. Sign of float not floating up is gasoline oozing out of nearly every crack and crevice in carb. And, of course, remarkably rich running.
Re-removed carb and studied situation. One corner of float drags against inside of bowl. Was it like that before? I did reassemble it exactly as before . . . so for the safe of smooth operation I filed a little off the corners of the float. Now it clears the inside of the bowl neatly.
Re-assembled and re-installed again. Faultless operation.
Thanks to everyone who offered suggestions and test procedures. I now know a little more about carbs.