How does the "Reserve Petrol Tank" Work In Bikes?

In Most of the Bikes,we have a provision for RESERVE TANK  to help the rider to go to nearest fuel station when the main tank runs out of fuel. Some might misunderstand that reserve tank is a separate tank. 

But actually, your main Petrol tank and your reserve petrol tank are the same tank. The only thing that differentiates 'main' from 'reserve' is which standpipe is being used to collect fuel. When your petcock is toggled over to 'Main,' a taller standpipe is being used, and so when the petrol gets below the level of this taller standpipe, the bike putters out and dies. Then when you switch over to 'Reserve,' a shorter standpipe is used to draw fuel from the tank. Thus, the bike will start up and run for a while longer.

Here's a crude little picture I made to show how this works:
 
 

This provision is used in bikes which do not have fuel gauges to show fuel levels.The reserve capacity of a fuel tank may vary from 10-20% depending on the manufacturer.

We can calculate mileage of a bike by measuring the reserve to reserve distance traveled and fuel consumed.Mileage depends on various parameters of which the road traveled is one.

What Exactly Is a Petcock?

 The petcock is a 3-position valve that changes the fuel source. The 3 positions are Off (switch is pointed towards the back of the bike), On (switch is pointed straight down), and Reserve (switch is pointed toward the front of the bike). The fuel petcock is located on the left side of the bike, about where your left knee would be while riding, above the frame, hanging off of the petrol tank. The lever is shown in each of the 3 positions below:

 
Petcock In On Position
Petcock In Off Position
Petcock In Reserve Position

The petcock also contains a secondary valve that is vacuum activated. Fuel can only flow when there is a vacuum from the left carb. applied to the nipple on the bottom of the petcock. Fuel exits from the nipple on the back of the petcock.

• blue line traces fuel flow from reserve

• red line traces fuel flow in normal position

• fuel exits back of petcock while vacuum line connects to bottom of petcock

What is the "Off" Position Used For?

Any time your bike is going to be parked for more than an hour, you should switch the petcock to 'Off,' according to our beloved FOG. The petcock is somewhat failure prone. If the petcock is left 'On,' and it is leaky, then it may leak fuel into the carburetors, and then, imperfect float valves in the carburetors will leak fuel into the engine, through the intake valves, and into the crankcase. Which is bad. Turning the Petcock 'Off' when you're not riding is a safety measure which may prevent this from happening if your petcock develops a leak.

This video shows how to disassemble the petcock for a rebuild: 

Out Of Petrol: Two Final Notes

Two final notes on running out of petrol:

First of all, here's a scenario that has happened to most of us. You're out riding, minding your own business, on 'Main,' when all of a sudden your bike putters out and dies. No big deal, right? Switch over to reserve, and crank it to be on your way. But the bike won't start.... 2 minutes later, all of a sudden it starts up properly and you're off. Why the delay?

When you run completely out of petrol while on 'Main,' the carburetors are sucked dry of fuel. It takes a little bit of time (or a little bit of cranking the bike) to get fuel sucked back through the carburetors. So next time you run completely out of petrol, just sit for a few minutes before you get worried, and then once you've given the fuel a few minutes to get through all the petrol lines, through the carburetors, and to your engine, you'll be good to go.

And, lastly, another scenario that has hit an unfortunate few of us. You're riding. Hit reserve, switch over, keep riding, no big deal. But there are no petrol pump (bunk) in sight. Finally you burn up all the fuel in 'Reserve' and you're still not to a petrol pump (bunk). Well, don't panic just yet. Often there is a little bit of leftover petrol which is not accessible to your Reserve standpipe, because it is on the other side of a hump in the petrol tank. Lean your bike as far over to the left as you can without dropping it (that should be, really really far), and then go back vertical again. If you're lucky, you'll now be able to make it a few more miles, and if you're really lucky, you might even make it to a petrol pump (bunk). The below image depicts this 'leftover petrol.'

  

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