IIRC: The throttle cable goes through the snout, through the cap, through the center of the spring and goes through a keyhole slot in the throttle slide. A lead "blob" on the end of the cable goes through the keyhole and the pressure from the spring holds everything together. The needle rests in a hole in the center of the slide - it just sits there, nothing holding it in place except gravity. The needle has slots cut into it so it can be adjusted up or down as necessary.
Correct. I've got it assembled so that the needle is trapped between the slide's floor and the washer that holds the throttle cable end (the washer is held in place by downwards spring tension). The circlip on the needle is sandwiched, so that the needle must travel up or down according to the slide's position, which is dictated by the throttle and return spring. Didn't move the circlip on the needle at all.
Took it apart again today and cleaned it out a second time. I'm going to go ahead with the tank de-rusting and lining process that RKL suggested. While draining the tank, it seemed to me that flow was a little constrained but still more than sufficient for this bike. It sips fuel. It did seem while spraying out the various passages that the float valve fuel inlet was pretty sticky. Sometimes I'd get spray returned back out the inlet port and the carb cleaner's hose wouldn't push past the float plunger, other times it would, and the bowl would fill with carb cleaner. I disassembled the bowl and swapped the floats and float plunger with a spare set I had purchased. Maybe that was the culprit, maybe not.
One other thing- some bikes have a On-Prime-Reserve petcock- the on position being vacuum activated with a diaphragm- if that is leaking air the fuel flow will be interrupted- check by seeing if it will run OK on reserve position.
The initial problem cranking over may have nothing to do with the subsequent bogging down- except in a related way- for example if the tank was removed, maybe the vent line got pinched on re-installation. that sort of thing.
Good grief, I had no idea that a fuel petcock could be so complicated. I rather doubt the one here is like that... I don't see a need for anything more than a simple gravity path from the tank to the carb 4 inches below it, with the ON position having a higher pickup than the RESERVE position. The tank is a typical dirt bike tank with a screw cap with an air nozzle and hose for pressure equalization that is threaded down into the cavity of the steering column and left to dangle. I don't see how anything could be vacuum activated in a tank that is never sealed.
