(1) What are the factors that can cause an engine in a water-cooled vehicle to overheat?

The thermostat has failed and cannot control the coolant temperature.

The cooling system is blocked, and the coolant cannot circulate effectively.

The coolant level is insufficient or there is a leak in the cooling system, resulting in too little coolant.

The temperature control switch and fan motor are malfunctioning, resulting in poor cooling performance.

The pump's suction capacity has decreased, resulting in insufficient coolant circulation.

(2) Under what circumstances will a water-cooled motorcycle break down?

(1) The air-fuel mixture is too lean.

If the carburetor’s air-fuel mixture ratio has already been properly adjusted by professionals using specialized equipment at the factory, the user does not need to make any adjustments. However, if it is determined that overheating is caused by an excessively lean air-fuel mixture, then the carburetor’s adjustment screws should be appropriately adjusted.

(2) Circulatory system blockage

Check the circulatory system for any blockages. The radiator should be cleaned with a radiator flush every 5,000 km, paying special attention to whether the small-circulation water pipes are twisted or blocked.

If the small cooling circuit is not functioning smoothly, once the engine starts, the coolant temperature inside the cylinder block and cylinder head water jackets will keep rising but won't circulate. As a result, the water temperature at the thermostat won't rise, and the thermostat won't open. However, when the water temperature in the water jackets rises above the boiling point, as molecular motion intensifies, the water temperature at the thermostat gradually increases, causing the thermostat to open. At that point, the high-temperature, high-pressure coolant flow from the water jackets rushes out through the radiator cap, leading to the "boiling over" phenomenon.

(3) The thermostat is malfunctioning.

The seat cushion and side panels are coverings originally designed for forced-air-cooled engines procured from the market. Their thermal insulation performance fails to meet the required standards, often leaving riders feeling that the seat cushion and partitions are excessively hot. This phenomenon is particularly noticeable in summer, leading riders to mistakenly assume that the engine itself is overheating. The thermostat’s function, after a cold start, is to reduce the flow rate of coolant, enabling the engine to reach its optimal operating temperature (approximately 80℃ to 95℃) as quickly as possible.

A genuine wax-type thermostat should start to open at a coolant temperature of around 70°C and reach full opening at approximately 80°C. If the thermostat fails to open properly, it will inevitably lead to poor circulation and engine overheating. Additionally, a malfunctioning radiator cap or insufficient coolant in the auxiliary reservoir can also cause inadequate coolant flow in the cooling system, resulting in overheating. Other causes—similar to those found in air-cooled engines—are not discussed here in detail. In fact, as long as a water-cooled engine doesn't experience "boiling," it will never overheat. Even when the coolant does boil, the cylinder head temperature will not exceed 120°C, which is significantly lower than that of typical air-cooled engines.

(4) Insufficient coolant or coolant leakage

When the engine is cold, open the coolant filler cap next to the radiator and check whether the coolant level is sufficient. To top up the coolant, do so at idle speed through the filler opening until the coolant tank is full. The coolant in the reservoir tank need only be topped up to about two-thirds of its total capacity. Also, check whether the engine oil has emulsified or deteriorated. If the oil has turned milky white due to emulsification, it indicates that there is an internal coolant leak. In this case, the engine must be disassembled to identify and eliminate the source of the internal leak.

Generally, internal leaks occur primarily at the joint between the cylinder head and the cylinder block. Replacing the cylinder head gasket can resolve this issue. The proportion of coolant to be mixed depends on the region of use and the concentration of the original coolant solution. The following table uses Honda Super Coolant as an example. In addition, carefully inspect each water pipe connection for any signs of leakage or contamination, check whether the water pipes are damaged, and verify that there is no leakage from the pump’s sealing holes.

(5) Valve clearance is too small.

The valve clearance for the exhaust valves is required to be 0.10 mm. However, due to dimensional tolerances in domestically produced engine components or because some users are intolerant of valve noise, many domestic manufacturers adjust the engine valves to very small clearances either at the factory or upon delivery to the user. This results in insufficient valve closure, which can lead to an extended afterburning period during the combustion process. A significant portion of the heat generated during this afterburning phase is used to raise the temperature of the engine rather than being converted into useful work, thereby causing the engine to overheat.

To ensure engine performance, valve clearances must meet certain specifications—not the smaller, the better. In fact, as long as the valve clearances are adjusted according to the requirements, a slight valve ticking noise will not affect normal operation. We’ve conducted comparative tests before: when valve clearances were adjusted according to specifications versus when they were set extremely small, the difference in maximum engine power was nearly 30%.