Yamaha DTR 125 L reg ?

You don't put water into the rad, use coolant. The capacity should be given in your owner's manual or if you can't find out then start by topping up the rad from empty with a litre of coolant. Run the engine til warm with the radiator cap off to dispell any air in the system & top up the rad til it's full. There's probably a bleed screw on the barrel or waterpump which will also have to losened to let air escape. When the system is air free, screw the rad cap back on, then top up the expansion bottle to it's maximum mark.

Steo
August 2012