For what it's worth, after reading what you all wrote, I had to go double check what was written on my petcock. I have Off, On, and Res. Off is off as I can run my carbs dry. On is vacuum controled. And reserve works as it is suppose to when I run out of fuel in the On position. Giving me enough to find a fuel stop to refill. I'm not sure about the Pri as I do not have that on my petcock.