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Greetings all. I am the proud new owner of my first vintage bike. I just picked up a 78 KZ650 from the original owner. Got some stuff to do to it and figured this would be a great place to learn. I'm sure I will have plenty of questions, some of which may be dumb. LOL
Like this, my petcock doesnt have an off position . It has run, res, prime. Where should I set it when not riding? Thanks in advance, Clint

Your bike has a vacuum petcock which is kind of an automatic fuel valve. Just leave it set to the ON position.
When the fuel gets low & the engine starts sputtering from lack of fuel, set it to RES (reserve), then fill up the fuel tank asap, certainly within about 15 miles or so. My bike has a low fuel warning light, so before the engine starts sputtering, I switch it to RES when the warning light starts to glow.

ON & RES. When the engine is running, engine vacuum actuates the petcock diaphragm to allow fuel to flow by gravity.
When the engine is stopped, a spring returns the diaphragm to the closed position.
ON or RUN is the normal position & it has a tall pickup tube.
RES is the reserve fuel supply & will use up the remaining 1/2 gal of fuel in the tank or whatever amount your reserve is. see owner's manual.

PRI (prime) bypasses the diaphragm assy to allow fuel to flow by gravity >> engine running or engine stopped.
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Thanks for the info, Martin. I've had bikes with carbs but they all had ON, OFF, and RES. I was confused. LOL
Look forward to getting this bike back into riding shape.

Petcock diaphragm failure is kind of common or at least a possibility. Some guys don't want to fuss with it, so they switch to a manual fuel valve. I prefer the vacuum petcock, but I keep a couple of automotive rubber vac caps on the motorcycle as a just in case >> then if needed, disconnect the vac line, cap the ports & ride the bike on PRI >> when the bike is parked, switch it to ON as the diaphragm plunger usually still shuts off the flow of fuel, even if the diaphragm itself is bad. Sometimes when the diaphragm craps out, the motorcycle will run terrible. mine did one time & fuel was spraying out the side of the diaphragm.

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So hopefully since I left it in Prime, I dont have a garage full of gas when I get home
The bike has been sitting for a couple of years but I havent seen any gas dripping yet.