UPDATE:

Got home from work and gave it a try, in neutral, left choke in, didn't touch throttle:

started right up, starter clicked maybe 3 times. probably 10 degrees or so (low 50s) warmer than it was this morning (low 40s).

I had left it on prime all day while I was working. Adjusted the idle a bit... took off. After I got through a few red lights I opened 'er up, and switched to reserve (i normally ride in res. tends to cut out less that way). Got through one stop fine, but cut out at the next stop. Put it back in prime.... rode it home fine.

Now that the idle is higher (and louder) I made a point to sit in prime, on, and reserve... and noticed a significant and immediate (though gradual) decrease in the engine when it was in on or reserve.... seems to just slowly cut out.


So....
1) battery is still my first thought. it could just be that this old battery doesn't like the cold and it was JUST warm enough to work.
2) i don't plan on giving it gas when i try in the morning.
3) definitely a problem with my petcock, right?
4) is it ok to just ride in prime until I figure this all out?


to be continued... fingers crossed for the ride to work in the morning (forecast calls for another freezing cold morning!!)

If you leave it on prime all day while at work, there should be a puddle under the bike at night when you are ready to go home.

As a vacumn assisted system, fuel will stay in the tank when the petcock is on run, or reserve as   long as the bike is not rumming.  However, in prime you are relying on gravity alone and gravity doesn't take the day off while you work and doesn't care if the engine is running or not..

The " choke" is an enrichener, and as such doesn't reduce the air portion of the air/fuel mixture (as a choke would) but rather puts more fuel into the mixture.  This is why when you pull out the enrichener, you should not turn your throttle when starting it.  That will flood the system.

Battery sounds  suspect, fuel lines are old enough to need replacement.
Have fun and let us know how the beater search goes.

Start bike, run on prime. Flip to on, run till it starts cutting out, shut off, drain carb bowl, see how much is in it. Close the drain, put it on prime a while, set it back to ON, drain again, compare volumes, See if youre starving,,

If it cuts out when on or reserve and not on prime, there is a problem somewhere in the vacuum operated petcock delivery system,.....most likely being the vacuum delivery to the petcock.

Running it on "prime", and parking it on "on", is exactly the same as running a manual petcock on "on", and parking it on "off", except you don't have a reserve.

Leaving it parked on prime, is the same as not turning a manual petcock off. It should not hurt anything,......but if it does, you better get it fixed....no matter which petcock you're using !

You could have more than one problem, each making it worse by itself and all together... killing it.

You're doing the petcock and seafoam thingy... have you checked your charging voltage?

And if low voltage is dropping your sparking voltage... maybe a new plug.