petcock help

OK, I get to give this a try and then I will be graded by the folks who know for sure. Wait for their answers before you take the info out of your RAM and retain it.
I think ON means that fuel flows from the tank into the carbs. But ON is only on when the engine is on and the vacuum goes up that little line and sucks some sort of diaphragm valve open (ON).
RES is for when you run out of gas because the pipe feeding the vacuum operated valve is at a higher level in the tank than the fuel level. Moving the petcock to RES accesses a lower opening which allows fuel to flow by gravity into the carbs so you can go directly to a fuelling station.
I think PRI stands for prime which is where you turn it if you are totally out of fuel and need to fill the carbs up before moving the petcock back to "ON". Why one wouldn't just use RES to accomplish this is beyond me.
Hope I passed.

 

Not 100% but I think reserve is the same as on(vacuum) just lower feeding filter inside tank. The prime is free flow as stated. Again, not completely sure but I believe this is the way the original Petcocks were designed.

 

Sounds right. I would give myself 2 out of 3 and half marks for the other 3rd = 83%.
I think that if you had it on gravity feed with no vacuum valve to shut it off then you would be relying on something in the carbs to keep the gas from just filling the bowls and overflowing into somewhere. Lawnmowers seem to have gravity feed tanks with no vacuum actuation. I assume the carb designs needed to make a bike engine go to 10k are configured differently from lawnmower carbs. I'd love to know how though.

 

If the original petcock is working, you just leave it on "On". That's the reason for the vacuum petcock- just leave it alone unless there is a reason to do otherwise. That being said, mine went bad and I replaced it with a good old "On", "Off", "Reserve" manual petcock.

 

just bought one of these bikes as a project and your absolutely right. making something difficult that dosnt need to be.