[Charlie D]

Petcock has a vacuum line attached at forward side so until vacuum is applied the valve is closed.

So check the lines as posted above.

 

[Charlie D]

Now we know the valve works.
Now you opened drain on carb and just a drop.
This says the float valve or the screen filter where the main line attaches to the carb.

So you will need to check the screen and in my opinion you need to take bowl loose and remove jets and clean them.

 

[Charlie D]

The chugging may be the air cut off valve stuck