Does it say anything in the bike manual? The manufacturer?

The fuel tank is usually located above the engine. If any parts of the fuel system have a small leak, you will lose fuel, including maybe into the engine. Shutting off the valve prevents this, unless the valve itself also leaks. In most cases, it won't matter, but nonetheless a good precaution.

"leak until you have no fuel" in the tank. It'll all be in the crankcase.

I always used the shutoff valve on my dirt bikes, but I don't remember using it on my street bikes.

GA to you.

No, most motorcycle carburetors have an overflow tube in the float bowl if the fuel level gets too high and the excess gasoline comes out a little rubber hose stuck down between the swingarm and the engine case (for most bikes). They are there just for that reason.

Some motorcycles won't flow fuel at all unless engine vacuum pulls a diaphragm in the petcock and opens the flow. My Suzuki DR350S is just like that. The petcock also has a Prime position that overrides the vacuum feature so you can start the bike if the float bowl is empty. This feature means that you don't have to worry about shutting off the fuel when you stop the bike. However, it does make it difficult to run the carb dry when the bike sits dormant and you don't want varnish build up. Alternatively, I just open the drain screw at the bottom of the float bowl and have a couple ounces of gasoline pour onto the ground.

If your petcock has an OFF position, it's usually a good idea to use anytime you stop for more than a few minutes just for the reason stated previously regarding crud, rust, etc. preventing the float bowl needle valve from seating properly.

Cheers and keep the rubber side down !!

Does your bike have an automatic vacuum actuated fuel shut off?

Usually it is because it will leak thru the carbs. If u have fi it might not be necessary

I've heard of people having their fuel all leak out through the carb when the bike was parked with the fuel tap on, but have never had this happen to me. IIRC, it was bikes with vaccuum-operated fuel taps that were likely to have this problem. OTOH, if your bike does not suffer from this problem you might well wear out your fuel tap prematurely if you keep turning it off when it does not need it!

As others have mentioned, the main reason for shutting the fuel of is to keep it from leaking out or into the crankcase or forming a hydraulic lock in the cylinder. New motorcycles often do not have a manual shut off because the shut off is vaccum actuated. On older motorcycles, there is a shut off that has three positions; off, on and reserve. If you have this type of manually controlled valve, shut things off. Petrol is too expensive to waste.

As for having a problem, I did have this happen twice. Once was on an old BSA Lightning. There was gas all over the place. I suspect that one spark and there would have been one good weiner roast. The other one was on a Kawasaki 500 and the fuel line cracked. If I hadn't had the fuel off, I would have had an empty tank and not made it to the deaership for a new line (taping a line up does work--for about 20 minutes).

Every bike I've owned that has a fuel gauge has not had a shut off valve. Perhaps the function of the valve is more to allow one to switch to "reserve" when the tank is running low?

I've never heard of switching the valve to off when parked.

My road bike never gets shut off(daily work commuter). My off road bike always gets shut off. For some strange reason the off road tend to "drool", on road not Both with carbs

The petcock is there for two reasons , it allows switching to reserve when needed and also it is a feature from way back when carbs. had a tendancy to leak for reasons described in your answers from your friends at CR4.

The primer buttons mentioned where notorious for leaking on my old Norton.

Hi Signode,

The Mikuni carburetor has an o-ring that if not proper will allow gas to seep out. Also any dirt on the needle & seat will allow a gas leakage. These happen because the gas tank is above the carburetor. I shut off my petcock every time on my 1970 Yamaha (that I bought new), and it has never worn out.

Another reason that people have told me to turn off the petcock is to hamper a thief. If he gets the motorcycle started, he won't get far before he runs out of fuel. If you are close by, you may be able to catch him on foot before he figures out the problem and gets it started again.

-S

You have been given all kinds of good reasons, some correct some not. As further encouregement, think for a moment what will happen if your float gets hung up or needles crapped up due to dirty fuel. The gas will form a puddle on the ground and the first passerby that drops a cigarette... Foom... your bike is now toast. Just do it...

all the other replies are very interesting.

From personal experience, some 20 years ago. With my little yamaha rd50 2 stroke, if i neglected to turn the fuel off, some would leak into the combustion chamber and flood it.

This would usually have to be rectified by having to shut off the valve, remove sparkplug, kick it over a few times to remove excess fuel, dry the spark plug, replace and then it would start. 8-)

An inconvenience but no catastrophy. although I did once have it leak out the overflow into a small puddle as described above.( onto tarmack which didn't do it any good at all).

this would ALWAYS happen if I left the valve on when I filled up aswell 8-(

so I agree with genereal concensus, the small inconvenience is worth the moments of panic, as you fumble for the valve when you realise you've forgotten to turn it back on 8-D.

There have been many possible reasons stated for " turning off your gas " when parked for a period of time . I have been riding since I was very young and remember when the practice came into play , when 2 strokes became popular . It was said that the oil mixed in the gas would settle into the float bowl if the petcock was left on and cause fouling of the spark plug when re-started . On the 2 stroke you were also supposed to shake the bike to re-mix the oil before turning the gas back on . As for the crud in the tank , it's going to settle to the bottom of the tank ( where the valve is ) anytime you bike sits for any length of time . If the valve is off , the crud will be sitting there waiting to flow thru as soon as you turn it back on . Gas takes much longer than a few hours to turn into varnish and unless you drain your float bowl after you turn the valve off , it will do that anyway . Also , unless you drain you float bowl all the seals are still subject to the effects of being in the gas . I still ride regularly and on all of my 4 stroke engine powered bikes , I don't turn off the gas unless I don't plan on riding it for a while , on the 2 strokes I do turn off the gas even if it's going to sit for just an hour or so .