A fuel gauge is still a rare find on motorbikes. Like, is there no way to know when your bike is going to cut off? The first time you lose power going 70mph with traffic barreling down on you, panic sets in. So, what do you do?
Instead of mapping out an escape route, switch to the reserve fuel and get to the nearest gas station. This motorcycle reserve fuel switch guide is for you if you’ve never heard of the reserve switch. We’ll discuss what each of the settings on the fuel valve does and when to use each.
What’s the Reserve Fuel Switch?
The reserve fuel switch is a three-way valve known as the fuel petcock. This switch has three settings; ON, OFF, and RES, and you will find it on the left side of the bike, just below the fuel tank.
The fuel switch controls the fuel movement from the fuel tank to the carburetor and engine. Furthermore, it lets you manage fuel using the three different settings.
The ON switch is what you should have your bike set when riding. On the other hand, the OFF switch stops fuel flow from the tank to the carburetor and engine. The RES feature relates to the reserve fuel. When you set the fuel switch to RES, your bike will use the fuel in reserve.
The reserve is part of the main tank only that the main outlet that transports fuel to the engine cannot reach it. The RES tube is longer and can reach fuel in the lowest part of the tank. So, when your bike cuts off, it doesn’t mean that the tank is empty. It’s just that the main tube can’t reach it. This is where the RES position comes in, as it allows the longer fuel tube to reach that last 20% of fuel so you can continue your ride to the nearest gas station.
Fuel Switch ON Position
You should have the petcock set to ON for regular rides. This setting allows 80% of fuel to flow to the carburetor and the engine. This works by having the main tube a few inches higher so it cannot reach every drop of fuel.
This is the position that allows the free flow of fuel. As such, the proper fuel-air mixture can happen. The combustion of air and fuel is what keeps your motorcycle moving. Therefore, every time you refill the tank, it is necessary to return the petcock to the ON position if it was on RES.
Fuel Switch OFF Position
At the OFF position, fuel cannot flow to the carburetor or the engine. Therefore, fuel and air combustion cannot happen. It is usually a safety precaution since it prevents fuel from reaching the carburetor.
The carburetor has a valve that allows fuel to flow in when the fuel declines. When you park, and the fuel switch is still in the ON position, fuel will start evaporating. However, parts of the fuel that are not as volatile will remain and turn into gunk. Eventually, the gunk will crust on the carburetor’s float valve, and the float valve will not shut off completely. Hence, more and more fuel will enter the carburetor and flood it.
Note that your motorcycle cannot start when the fuel switch is in the OFF position. The OFF position is also ideal out of season or when storing the bike for several days. Most riders never use the OFF switch, but it is recommended to do so occasionally. This can prevent the carb from getting sticky from fuel varnish sooner.
Reserve Fuel Switch RES Position
The RES position is the reserve fuel setting. It allows the bike to reach the reserve fuel and flow to the carburetor and the engine. If you run out of fuel when riding, just reach for the petcock and switch to RES. Since the petcock will be in the ON position, turning it over 180 degrees will bring it to the RES position. Your bike will spit and sputter for a bit and then fire back up.
You should not get into the habit of leaving the petcock in the RES position on regular rides. Doing so means that when the motorcycle cuts off, your tank will be empty. The reserve has about 10 to 20% of the fuel, so it’s not much, especially if you’re on an adventure bike on the trails.
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Frequently Asked Questions
How long can you ride on reserve fuel?
The reserve fuel should give you 20 to 30 miles on the road. Thus, it’s best to use it to find the nearest gas station. The reserve fuel is not a specific amount, so you can’t know the exact amount of fuel left to estimate the exact distance you can travel.
What is the ‘PRI’ setting on the petcock?
Vacuum petcocks have the ON, RES, and PRI settings. The petcock on such bikes operates via a diaphragm which shuts off fuel flow when the engine stops. In addition, a hose connects the petcock to a fitting on an intake manifold in one carburetor. This mechanism supplies vacuum to one side of the diaphragm, thus opening the fuel switch inside the petcock.
The PRI setting stands for Prime. It allows the petcock to bypass the diaphragm so fuel can flow into the carburetors when drained off the gas. The PRI position is important when your bike has been in storage or tipped over. Fuel enters the carburetor so you can start the bike when the engine is not running. Once you use the PRI setting, you should switch back to the ON position.
Do all motorcycles have the reserve fuel switch?
Not all bikes have a fuel switch. It does not necessarily mean that the motorbike does not have reserve fuel. On such motorcycles, you may have a low fuel light or fuel gauge that will tell you’re running low on gas. Nevertheless, if your motorbike has neither of these features and there’s no fuel petcock, likely, it does not to have reserve fuel.
Conclusion – Motorcycle Reserve Fuel Switch
There you have it, guys. The reserve fuel switch serves as a low fuel warning. When you switch to the reserve fuel, you’ll have only a few miles of travel. Furthermore, do not run your bike on RES all the time to avoid being stranded when the tank empties without warning. On the other hand, no worries about refilling the reserve tank since it’s not a separate tank.
Now you know how not to run out of gas by using this reserve fuel switch guide wisely.
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Sam is Automole’s editor-in-chief and classic car enthusiast. Sam is studying mechanical engineering at Cockrell School of Engineering, Austin. He also writes for many top automotive publications and appears on the Collecting Cars Podcast.