A rusty motorcycle petrol tank can corrode to the point that you get holes in it. Not only is it expensive to lose petrol through a hole, but it's also very dangerous. Don't risk getting to this point. Clean your petrol tank every now and then. It will protect the tank itself, and it will also help your motorcycle to run more smoothly and efficiently.
Remove the petrol tank from the motorcycle and safely dump out any remaining fuel.
Rinse the inside of the tank with water, and then shake the tank to remove as much water as possible. Add 711 to 948 ml (3 to 4 cups) of phosphoric acid to the tank, and shake the tank to coat the inside walls. Allow the phosphoric acid to sit in the tank overnight or up to 24 hours.
Dump the phosphoric acid out. Since phosphoric acid is used as a household cleaner, you can dump it down a drain. Rinse the tank with fresh water. If there appears to be deposits sticking to the inside walls of the tank, drop a few coins, bolts or washers into the tank and shake it. The loose metal should loosen up any remaining deposits that have adhered to the walls.
Dissolve 100 g (1/2 cup) of baking soda in 2 litres (1/2 gallon) of water. Pour the baking soda/water mixture into the tank and allow to sit for two hours.
Drain the baking soda/water mixture and rinse with fresh water once more.
After shaking as much water as possible from the tank, spray WD-40 inside the tank to prevent rusting.
Phosphoric acid can be found in the bathroom-cleaner section of supermarkets.
Make sure you properly dispose of any fuel that you drain from the tank. You can dump the old petrol in an empty plastic washing powder bottle and take it to a petrol station or spare parts shop that recycles oil and petrol.