Some models are equipped with a shutter-type valve in the exhaust manifold outlet. This valve is operated by a temperature-sensitive bi-metal spring. When the engine is cold, the valve shuts, directing hot exhaust gases back upward to warm the carburetor and intake manifold. When the engine warms, the valve opens allowing the normal passage of exhaust gases. A special heat riser lubricant can be used to keep the valve working freely. This is important, since rust and scale can form on the valve, making it stick in either the closed or open position. If it sticks in the open position, hard starting and poor drive-ability will result until the engine warms up. If it sticks in the closed position, the engine will suffer from loss of power, overheating, poor performance and poor fuel mileage.