12v means vehicle voltage, that can be battery voltage when engine is off, 12.3-12.8volts, or it can be 13.5volts to 14.9volts just after engine starts, when alternator is working to recharge the battery.
So not your problem.
Reads like you need to set warm engine choke position.
Look here: Carter YFA Carburetor Exploded View
#13 is the choke adjustment cap, there may be rivets holding it on but hopefully there are 3 screws that you can loosen to turn the cap
With engine fully warmed up, turn engine off but leave the key on
Turn the cap until choke plate is fully open, not too much, just until it is fully open, then tighten the screws
The reason "they" recommend using the stator voltage is because it only has voltage when engine is running, not when key is on, only when alternator is working.
For example if you were to turn on the key and listen to the radio for a few minutes before starting a cold engine, the Choke Heater would already be heated up so choke would be open more than it should be and engine would not run as well.
With stator hook up that can't happen.