What is the off position for the petcock in the vstar 250? I only see ON, PRI, and RES. I just got the bike from someone else and I know that you are supposed to turn the petcock to the off position when you're done riding. Thanks for the help!

res top, off to side, on bottom

Hi there are there three positions on the fuel petcock pri,on, res? and the fuel keeps flowing in all positions if so there is dirt or the o ring is no longer seating properly replace the fuel petcock diaphram or clean the petcock of any material not allowing it to fully close.

Fuel petcock position depends entirely on what you're doing with your bike at the time.

In normal use, the fuel petcock should be set to the "on" position and left there. In that position, gasoline will be drawn from the tank only when the engine is running and generating vacuum. This prevents overfilling the carbs when the bike sits.

When you're riding the bike and it starts to sputter, it's time to move the petcock to the "reserve" position so that you can use the last few drops in the tank (on a 650 special, this was either .3 or .5 gallons--can't remember which off the top of my head) to nurse your bike to a gas station. Once you've filled the tank, move the petcock back to "on." The bike will still run fine if you keep the petcock in the reserve position, but when it starts to run low on gas--well, you won't have that emergency reserve to get yourself to the gas station.

If you've managed to run the bike out of gas completely, or if you are getting the bike ready for service after having drained the carbs for the winter, you'll need to move the petcock to the "prime" position for a few minutes. This by-passes the vacuum line cutoff mechanism and lets gasoline flow until the carb floats shut off the fuel valve (this is also how all older motorcycle fuel petcocks operated). A sticky float or poorly sealing float valve in the carb can, however, cause far too much gasoline to drain into the carbs (and into the crankcase, if you're particularly unlucky), which is why you should not leave the petcock in the prime position for an extended period of time.

Some Yamaha XS650 motorcycles (can't tell if that's the model you have) have a tank with two fuel petcocks, one on each side, to fuel each carb. If you have a tank with this setup, be aware that setting one fuel petcock to "prime" will affect both cylinders, as there's a crossover tube to equalize fuel levels between the carbs. Also, for reasons I could never figure out, on one XS650 I owned, moving either petcock to "prime" permitted fuel to flow on a gravity feed out of both petcocks simultaneously. I could never figure out exactly why it was doing that.

You have a vacuum operated petcock....so that when engine starts and draws vacuum...a diaphragm lifts in the petcock to supply fuel to carb. When the engine shuts off...vacuum is lost and the petcock closes.
If you run the tank empty....in order to get fuel into the carb and get the bike started initially....you can open the petcock without engine vacuum by rotating petcock to the PRIME position. Once started return the petcock to the ON position. If you leave it on prime....with engine off...fuel may leak into carb and run into engine and contaminate crankcase oil with fuel.

What you are dealing with is a vacuum petcock with a bad vacuum diaphragm , in this case fuel is bypassing the petcock via the vacuum line and filling the engine (oil) with gasoline.
If you have a standard petcock that is leaking (not shutting off properly) or just left in the on position a stuck float needle will also fill your engine (oil) with gasoline. This can happen with a clean carburetor and a good float needle. It just sticks and you have to change your oil. If you have the kind of fuel petcock that allows you to turn it off when not in use , always turn off your gas when not riding.
You will have to change your oil if it is fouled with gasoline. I suspect a vacuum operated petcock is at fault. With standard petcocks I have seen new motorcycles in perfect order stick a float needle ( Kei Hin Carburetors ) and fill there engines and air boxes with gas/oil mix. On vacuum petcocks the diaphragm that shuts off the fuel flow is usually leaking and at fault.

( PRI ) is the primer position for the petcock ( fuel valve) . This position is used for a couple of reason. this kind of valve works off vacuuum from the carbs, but if you run out of fuel you or remove the tank you can't create vacuum, so putting the valve into the PRI position allows fuel to flow inton the carbs without vacuum so you can start the bike. Once the bike has started you should switch the valve back to the ON position ON= normal position, PRI= primer position, RES= reserve postion. I hope this helps!!!

There is no OFF, you have RES toward front , ON is all the way down, and prime toward back, PRI is used when it has been sitting for awhile. RES and ON wont let gas out unless bike is runnig. There is a vac hose on back of petcock that lets ON and RES work it must be connected..

Remove the petcock valve and dissasemble and clean it may have a clogged passage or this type may have bowl with a screen in side that is dirty, clean the valve with gum out and blow out with compressed air if possible.

The On position is where you run the bike after filling the tank. The Res position is when you are almost out of fuel and the motor stutters you turn the switch it it and that will give you about 1/2 a gallon of gas left in the tank to get to a gas station and refill. Pri is for priming the carbs only. If you have ran out of gas after using the reserve fuel you switch it to this position to fill the float bowls with fuel. Do this for only 30 seconds and switch it back to On position.