How does the reserve tank work?
A common misconception among new riders is that there is a "reserve tank" on a motorcycle which is actually a separate tank from the main tank. Some also worry that the reserve tank might run out of gas independent of the main tank, but they can't figure out how to fill it.
These pictures show you how the tank is actually set up. In the first picture, the yellow arrow points to the petcock intakes (or "straws" in the following discussion). The blue area is the "fuel reserve." The red area is unusable fuel. The white area above the blue, inside the tank, is the "normal" fuel. There are about four gallons available in the main part of the tank and .8 in reserve. The last photo is of the petcock.
There is no separate reserve tank. Almost all motorcycles have just one gas tank. It's the big metal or plastic bulgy thing with the filler cap on top that sits in front of the seat and behind the handlebars^. On bikes with petcocks^^ (like our Ninja 250s), there are usually three or four settings available^^^ (see below). On the Ninja 250 we have three: Off, On, and Reserve. On the Canadian ZZR250 there are four: Off, On, Reserve, and Prime.
The Off setting is pretty self-explanatory -- it shuts off fuel from the tank to the carburetors, and is how you want to leave the bike if it's going to sit for a long time. On the Ninja 250, this is when the long side of the valve handle is pointing straight back.
The On setting is where the petcock should be set for normal riding. This setting allows the fuel from roughly the top 80% of the tank to flow to the engine. The way it does this is by putting a tube a few inches up into the tank, so it can't reach the fuel on the very bottom. This is exactly the same as having a SuperDuperMegaGulp but grabbing the SuperGulp-sized straw: there's that last little bit that you just can't reach because the straw's too short. On the Ninja 250, when the long side of the petcock handle is pointed straight down, it's in the On position.
The Reserve setting is just like finding a longer straw. It allows you to get that last 20% of the fuel in the tank. It's not a separate tank, just a separate straw. Practice switching to reserve when you're riding -- it's quick and will save you a lot of frustration and danger compared to letting the bike die and having to crank it at the side of the road. Remember to switch it back to On when you've filled up the tank! Reserve is selected when the long side of the petcock handle is pointing straight forward.
The Prime setting (on some bikes, including the ZZR250, but not the EX250F sold in the US) is used to bypass the vacuum diaphragm that normally stops gas from flowing unless the engine is running. The vacuum diaphragm needs low pressure to allow gas to flow, which normally means that the engine needs to be running. The Prime setting allows gas to flow regardless of whether the diaphragm gets vacuum or not. This is very useful if your engine won't start right away because it's been sitting a long time, ran out of gas in the On position, or just fell over. Again, note that the EX250F (1988-07 Ninja 250) does not include Prime, just On, Off and Reserve.
Why is the bike set up like this? "Because" seems to be the best answer. Motorcycles (and even cars, many years ago) all work like this: when you get near the bottom of the tank, you "run out of gas" on the regular setting, so you switch over to the reserve setting when you feel the engine start to stumble. The syptoms are: First you feel that the bike is losing power. Then the engine starts to stumble pretty badly. Then it stalls completely. If you switch to reserve before the last thing happens, you'll be fine.
Once you've switched, you know you've got around 40-50 miles to find a gas station before the engine stops running. It's a very inexpensive and traditional "gas gauge". Most motorcycles don't include a normal gas gauge like modern cars do, for weight, complexity, or cost reasons.
It's best, when getting to know your bike, to use the trip odometer as a kind of gas gauge. Reset the trip meter to zero when you fill the tank, then take note of how many miles are showing when you have to switch to reserve. On the Ninja 250, it's between 200 and 250 miles for most folks, although other extremes are certainly possible. Once you know it's (for example) 200 miles, then you can tell that at 100 miles you've used about half a tank of gas. If you can do simple arithmetic in your head, you can also tell how many miles until you'll have to switch to reserve and plan when you need to fill up again. The trip odometer becomes a kind of gas gauge.
From above: ^ Some motorcycles have more than one gas tank, or have the gas tank in a "weird" spot, like under the seat. There's even one bike that stores gas in the frame tubes. Most motorcycles only have the one tank, and it really is the tank-shaped thing between the seat and the handlebars.
^^ Fuel injected bikes do not have petcocks, nor do some carbureted bikes. Motorcycles without petcocks usually have a "low fuel" idiot light, which is equivalent to hitting reserve. Some have car-type fuel gauges, which are known for their unerring inaccuracy.
^^^ The fourth setting on some petcocks is called Prime. This setting is detailed above. Note that some bikes only have three settings, but have Prime, On and Reserve, with no Off setting. Hooray for diversity!