Now Routy, come on now, our vac petcocks are NOTORIOUS for failing in 2-3 different ways over time -- didn't you know that?
This gentleman attempted to continue to use his stock vac petcock using the Prime trick for a while now (giving up his reserve function, but keeping the vac petcock body as a manual petcock).
He may be discovering a new vac petcock symptom that occurs when this Prime trick is done for a goodly amount of time, so we may all learn something new as we go through this one.
He may have discovered failure mode #4 ..... ==============
Seriously, now that I've finished teasing with Routy over his perpetual petcocky blind spot ....
First off, whack your float bowl 3-4 times smartly with a big screw driver handle -- this will jar free a stuck float and get you running again if this was you problem (yeah, I know -- but it is cheap enough to do and stuck floats do happen occasionally with folks using stock petcocks)
The main they refer to is the main jet, the one the needle runs through. Yes, folks have had them unscrew when they weren't put in with the correct torque from a previous tear down.
"Totally stock" likely means no fine paper pleated in-line fuel filter -- you may want to consider adding that while you are in there fiddling with the hoses, etc.
You may want to do this just to cut down on any future carb problems (especially if you find any sediment or trash when you go into the carburetor).
Another item you might want to check if you do take the tank off, is to unscrew the vac petcock unit and take a look at the screen -- I had mine varnish up closed and had to spray it with carb cleaner just to open up the little pores in the screen mesh so gas could get through.
fuel & spark -- spark is easy and cheap to check (replace the plug and check spark on the new one) Don't forget to clean all the sand and grit out of the spark plug hole area before removing the plug -- don't want any of it going down the spark plug hole to score up your cylinder walls.
My betting money is going on a carburetor problem, not spark. Spark doesn't come and go (it just dies) and it can't give you fuel/flooding smells ....
Float sticking can come and go and it can starve you if it sticks up, and it can flood you if it sticks down. I'd carry a big screw driver with you for applying the 3-4 float bowl whacks until you get this thing all worked out.