This is a long shot, but did you try riding with the gas cap open?
A clogged gas cap vent hole would show the same symptoms.

my guess would be a sticky float or crud in the fuel

Good ideas both... although I've *heard* of gas cap venting becoming an issue, I've never experienced it myself.

I just went cowtrailing for about an hour with my wife, so we didn't go fast, under 10 miles an hour. Like taking a walk but you don't have to do any walking haha....

Anyways the bike behaved perfectly well the whole time.

I did suspect a stuck float, as I have to make my own gaskets for this old carb, there are no more kits available. I use bicycle tyre rubber, and I pre-expand it in gasoline before cutting it to size, so that it doesn't expand and wrinkle after installation, causing a leak. It seems to work but I've had expansion problems in the past where it reached in and stuck my float. (by the way if anyone knows of a non-expanding, non corroding gas submersible gasket compound, I'd like to buy you a drink or shake your hand cause I've tried everything and asked everyone.) -at any rate, it's not the float or anything to do with the idle circuit, for sure, or it would have showed up on this evening's ride.

Crap in the gas, I'll check into that and it is possible, considering the ammount of rust in the tank. Original 42 year-old tank and it was clean and shiny as a whistle inside till my little brother left the cap off all winter last year, before I went home and got the bike back. I probably need to cream it, or at least put some nuts and bolts in it and shake it with some chemtool, but I did nothing. I think I shook it and sloshed the old gas around to knock loose things loose before dumping it out, and then it was back to business. I'll check into that theory though.

I'm also still suspicious of an electrical break that may be happening? I've already made some serious modifications to the electrical system. I've been so tired of this weak little 6 volt system. I took a schematic and figured I'd see what I could do to improve it. First thing was wiring up a toggle to the battery, so that I can disconnect the battery and run the spark off the charging system in case I ever get stuck somewhere with a dead battery. (can't jumpstart unless someone happens by with an old british 6 volt car?) Then upon studying the charging circuit I noticed that there is actually an extra phase in the alternator? this extra phase is automatically activated when you turn on the headlight. The headlight switch actually throws two switches at once, down inside the handlebar mounted housing. One for the extra phase, one for the lights. This is the stock system. I suppose this bolsters the charging system to handle the extra load, but decreases the surplus when not needed, to spare the delicate selenium rectifier.

Well now I've replaced the rectifier with some newfangled little thing that is about a hundred times cheaper and at least fifty times more robust. So I took apart the headlight switch and mounted that extra alternator phase switch to a toggle on the headlight cowl. I did this because the charging system is so weak, I often end up with a half dead battery at the end of a night-ride anyhow, and it will really help to run the bike for a short time after the next startup with the bolstered charging, but without the drain of the lights. What do you think?

I'm looking for a cheap small ammeter (largest possible diameter 1.75 inches) so I can more closely monitor, and manually manage my delicate little charging system. I may also install a volt meter but I'm not sure about that one yet.

I've also simplified a few other things to eliminate unecessary burden and draw. I'm tempted to replace all wires with new, thicker wires.

And at the very least, I just might solder all those damn bullet connectors. Maybe that will finally solve this mystery short?

(on another semi-related note, I seem to hit a rev limit before I redline or lean-out, or run out of available power. It seems to be lack of spark that keeps me from reaching that last little bit in the top. this is an 89cc four stroke, my wife and I seem able to get up to 55mph on a long flat, but a more reasonable top speed is about 40. I'm thinking it will do more if only I can reach that last little bit on the rpm and I think part of the key is eliminating electrical waste in the system. The spark quality on this bike is very sensitive to the overall system)