Suzuki gs750 petcock

There is no “off” position.

Quite honestly, I run my GS550 in the Prime position most of the time. I find that the vacuum “on” position doesn’t supply enough fuel for a spirited run through Deals Gap at high RPMs. (Even with a brand new petcock).
I switch it to “on” when riding to and from the play ground and when it’s parked.

Right now you haveAn on/off petcock. It is just that the Prime position is “on”, and the On position is “off”. The only risks you have are:
1. There is no reserve position because the RES and PRI positions draw from the same level. And
2. If the diaphragm fails, it will leak fuel out of the vacuum barb, but that would be a tell tail that the diaphragm had failed. You could put a vacuum cap on the vacuum barb, but you’d lose the tell tale. But the chances of a failure on a diaphragm that isn’t being used is small. It won’t be in contact with the fuel that deteriorates it.

The prime setting flows almost twice as much as the ON
setting. Test it. You’ll see. Put a hose on the vacuum port and suck on it. Let the fuel flow into a cup. Then compare it to the prime setting.

As I stated before, I have to run in the prime position on high rpm runs because the ON position can’t keep up with my fuel consumption

Yeah. I’m not talking about a jetting issue. On short runs, the bike runs perfectly, but during a 9 mile pass that is above 8k rpm about 80% of the time, it will start to starve for fuel slightly at around mile 6 if the petcock is in the ON position. No issue in PRI position.
this is with a brand new OEM petcock. I’ve even went to larger ID fill tubes between the carbs. It’s just flow issue with the OEM petcock.