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It’s mentioned in the user manual of my motorbike, that when not using the vehicle, I should turn the petrol tap (the one that has three positions “On”, “Off” and “Reserve”) to “Off” position. I don’t understand why that’s necessary (and I need to understand that for my own knowledge and because my brother won't do it unless I can give him a convincing reason)?

So three questions:

  1. Is it because petrol may evaporate or flow to the engine un-necessarily when the vehicle is parked and not used for a while (like overnight or just for an hour or two)?
  2. Is it recommended to always turn off the petrol tap whenever I park my vehicle somewhere or just when I park it in the garage for the night?
  3. Also, as an additional related question: Why do some people drive their bikes with the petrol tap always kept on “Reserve”? My uncle does it, but refuses to explain it.
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In the edit that is in moderation queue I used two terms that are more common in Britain (petrol tap), and US. (petcock). Correct me, if “fuel knob” is also legitimate and recognizable. –  theUg Jan 14 '13 at 21:25
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2 Answers

up vote 12 down vote accepted

The reason is that motorcycles traditionally have the fuel tank higher than the carburetor, and the fuel feeds with gravity alone. What risks does this introduce that necessitates a manual shutoff? Without the shutoff, if the carburetor float failed to close the valve tightly enough to stop the fuel flow, then gas would continue to trickle into the carb, overflowing the bowl and flowing down into the intake tract. If the intake valve were open, it would fill the cylinder. Then, upon attempting to start the motor, the incompressible liquid gasoline would cause a catastrophic hydraulic lock.

More modern motorcycles have changed some of this in a few ways:

  • the fuel petcock could be vacuum operated, closing with the engine off.
  • the fuel tank could be lower than the carbs, making a fuel pump necessary.
  • fuel injection has eliminated the use of carburetors and floats to control fuel flow.
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welcome! good answer! –  mac Jan 14 '13 at 19:58
    
Can you provide more information or sources on hydraulic lock and its universality? It certainly sounds plausible, but I had never heard of it being such big of a deal. As another answer points out, most worry about flooding the combustion chamber making it harder to start. –  theUg Jan 14 '13 at 21:44
    
It is often referred to as hydrolock. Here is one reference: ridingoutloud.com/what-is-hydrolock-and-how-do-i-avoid-it Note that article references vacuum controlled petcocks, and points out that while they are convenient, they can fail. –  Tim B Jan 14 '13 at 22:19
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The primary reason for shutting off the fuel is safety. On a motorcycle the fuel tank is directly above the engine. If fuel were to leak it would drip directly on the hot engine. This along with the fact that most motorcycles use a rubber supply hose that is exposed to the engine heat and the resulting decomposition. On an automobile the fuel is usually isolated from the heat and transfered from the tank to the engine via a steel line. The fuel shut off has three positions run,reserve and off. The difference between run and reserve is the length of the fuel delivery tube that brings fuel out of the tank. The "normal" run tube rises about 35mm from the bottom of the tank. This means that when it stops delivering fuel approx 35mm of fuel left in the tank. The "reserve" setting fuel pickup is at the lowest level of the tank. It will flow fuel until the tank is empty. Some riders feel that using the "reserve" setting uses the fuel at the bottom of the tank preventing water from accumulating which could result in the tank rusting.

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a slight addition to this: on many bikes, leaving the selector in the reserve position may cause fuel to drip out onto the floor continually until the tank is empty. –  Rory Alsop Jan 14 '13 at 8:49
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