Fuel tank cap for Viper 125-200 cc motorcycles and similar (China, Minsk). The cap closes the filler neck of the tank, protects the fuel from dust and water and keeps it from splashing out. Inside there is a vent hole or valve.
Replace the cap if the key is lost, the lock is worn, there are cracks and gasoline seeps out. If the tank does not "breathe", the engine may stall because of vacuum: in that case also check the cap valve.
The cap is plastic, keys included.
