The most common cause of overheating in these bikes is air in the cooling system. The air doesn't conduct heat to the thermostat efficiently so it doesn't open correctly and the bike overheats. There are a few things to check out.
When the coolant heats up it expands and is forced into the reservoir and when it cools down it's supposed to pull back into the cooling system. A worn/dirty filler cap or a cracked/leaking vent line will allow air to be drawn into the system rather than coolant.
The bleed screw is another area to check. It's easy to overtighten the screw and break the threaded metal insert away from the plastic housing and that will let air in. That's a non-repairable problem and you have to replace the entire thermostat housing.
The bleed screw also uses a copper washer as a seal and these harden with time and reuse. You should be able to find replacements at your local auto parts store.
Clean out the coolant reservoir. I found about an inch of rust in the bottom of mine, and that reduces the amount of coolant available for cooling system refilling. You'll need to remove the reservoir and blast it out with water pressure.
When you refill the system you need to remove the bleed screw and fill until the coolant is level with the top of the bleed hole threads. That's the actual system coolant level indicator -- not the area under the filler cap.
Also, if you top off the system use either silicon-free 50-50 pre-mix (I use the Honda motorcycle pre-mix) or distilled water. Don't use tap water as that will add minerals that will encourage corrosion.
On the electrical side of things, unplug the connector to the fan switch and short across the contacts. The fan should start up immediately. If it doesn't, check for debris in the fan shroud -- leaves, gravel, etc.
You can test the fan switch by dipping it in boiling water and checking the resistance across it with an ohm-meter. It should read infinity (open) at room temps and zero (shorted) when hot.
Also check that the radiator is clean. You'd be surprised at the quantity of bug guts that can build up there.
To clean it, remove the covers, put some regular drug store hydrogen peroxide in a spray bottle and spray it into the radiator repeatedly until the foaming stops. Then hose it out from the back. The hydrogen peroxide breaks down the bug squish proteins and makes the goo easy to rinse out.
Another cause of excessive engine heating is a lean mixture so check that your air filter is clean. If the bike feels 'weak' or there's a misfire at small throttle settings you may have crud buildup in your pilot jets. Try running some fuel system cleaner with a couple of tanks of premium gasoline and see if the small throttle response improves.
There's an annotated photo of the thermostat housing in my PhotoBucket album that might be helpful in locating the parts.
http://s279.photobucket.com/albums/k...0Head%20Stuff/
Jim
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Note: This post may have been altered without the knowledge or consent of the author.
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