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.

This picture shows you how the tank is actually set up:

The yellow arrow points to the petcock intakes (or "straws" in the following discussion). The blue area is the "fuel reserve," and is accessed by the short tube. The red area is unusable fuel. The white area above the blue, inside the tank, is the "normal" fuel, and is what the longer tube can reach. Each tube has an opening at the end, preventing it from reaching the fuel which is lower than its upper end.

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***. On the Ninja 250, we have three: Off, On and Reserve. On the 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. Remeber 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 (only on the ZZR250 -- if you're living in the US, you have an EX250, not a ZZR250) 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 EX250 (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 worked 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. 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 (I've had my bike hit reserve at 180 miles, but that's unusual). 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.

* Some motorcycles have more than one gas tank, or have the gas tank in a "weird" spot, like under the seat. I think 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 a car-type fuel gauge.

*** The fourth setting on some petcocks (on the ZZR250, but not the Ninja 250, alas) 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!

Created by Ian Johnston. Questions? Please mail me at reaper at obairlann dot net.