The 1985-1988 Honda CH250 is a liquid-cooled four-stroke 249-cc engine. The engine is  primarily cooled by a radiator in which heat from the engine is carried to the radiator by antifreeze/coolant. Air passing over the radiator coils removes the heat from the engine. A radiator fan assists cooling at high temperatures.

The switch on the radiator is the thermostatic fan control switch. When the coolant gets hot enough, the fan comes on. To check its operation, remove fan plug from radiator switch and jumper wire the plug with the ignition on and the fan should run. That will tell you if your fan is good. The thermostatic fan switch has continuity at 208 degrees which means the fan comes on at that temperature.

Your thermo sensor housing is on the throttle side of the bike just above the oil
drain plug. The thermo sensor, which sends information to your temperature gauge, has a single wire plug attached. To check your temperature gauge operation, remove wire and ground it with the ignition on. Your temperature gauge will move all the way to hot. That will tell you if your temperature gauge is operational. 

The thermo sensor is a switch which reads the temperature of the coolant at the engine. The thermo sensor is checked by removing it from the engine and placing it in hot oil and reading the temperatures. 

Your thermostat is in the housing below the thermo sensor. Remove it for checking and replacement. Be advised that the thermostat begins to open at 208 degrees, which is considerable. It can be very hot and the fan will not always be running. 

Honda’s Service Manual recommends that  coolant should be changed every two 
years. As coolant ages, it loses its ability to resist boiling and conduct heat. But, more importantly, it also loses it anti-corrosion properties, and this will allow the build up of scale and residue in the cooling system. 

This build up will reduce the cooling system's ability to do its job and could eventually render the system useless. To prevent this oxidation of the aluminum cooling passages, you should replace your engine coolant at least once every 2 years.

Motorcycle Consumer News investigated a rash of water pump seal failures in Honda Gold Wings and concluded the cause was from silicates found in regular automotive antifreeze. Honda recommends using their own green coolant which is free of harmful silicates.

Conventional automotive antifreeze contain silicates (sand) which scrubs the radiator core to keep them flowing and free from obstruction. Automotive water pumps contain seals which can withstand the silicates. Honda’s motorcycle and scooter water pumps do not have that luxury. Be certain to use Honda motorcycle coolant or automotive coolant equivalent that is silicate free! All coolant should be mixed at a 50-50% ratio of coolant 
and distilled water.

Tap water, which varies dramatically in areas, contains ionic species of calcium, magnesium and chlorine. If there are any significant ions present, you can generate bimetallic corrosion within the metallic components of the radiator. Distilled water is a poor ionic conductor (ions removed), as is the components in the antifreeze. So bimetallic corrosion is minimized (weaker electrolyte). Most antifreeze will have additives in it to reduce the corrosion. If you flush the coolant at the recommended intervals (even with tap water), you are unlikely to have any problems.

The coolant check procedure should be done with a cold engine!  Place the scooter on its center stand. To remove the front cover lid exposing the radiator cap, use a Phillips screwdriver to back off the cover screw while using your fingernail or other small pry underneath the screw lip. Slide the cover up toward the headlight and remove it. Start the engine. Check the radiator coolant level in the reserve overflow tank next to the radiator. At normal operating temperature, the level of the coolant in the recovery tank should be between the “upper” and “lower” line marks.

The coolant change procedure should also be done with a cold engine. Place the scooter on its center stand. Unscrew the trim clip screw and slide the front cover lid toward the headlight and remove. Loosen but do not remove the radiator cap. Place a drain pan under the water pump and remove the drain bolt. The drain bolt is the 10mm bolt with the aluminum washer nearest the bottom of the pump cover. Coolant will exit the drain bolt hole slowly. By unscrewing and slightly lifting the radiator cap, coolant discharge control can be maintained.

To flush the system, replace the water pump drain bolt. Fill the radiator with water and start the scooter. Run scooter until warm. Stop the scooter. Remove drain bolt and release water. Replace drain bolt.

To refill the radiator, Fill the system through the radiator cap hole with a 50-50% mixture of distilled water and ethylene glycol coolant. Do not use alcohol based coolants. Start the engine and run until there are no air bubbles in the coolant, and the level stabilizes. Stop the engine and add coolant to the proper level if necessary. Reinstall the radiator cap. Check the level in the reserve tank and fill to the proper level if necessary.

Take the scooter for a ride to get it up to normal operating level. Check for any leaks.

Note: Dispose of the coolant in a safe manner. Ethylene glycol tastes sweet but is very deadly to animals which may come along and drink it if you drain it onto the ground. It is equally toxic to children which are attracted to its lovely color.

Randy Pozzi (Rev. 08/2006)