Assuming you have checked the radiator is clear my thoughts of things in priority order to check before you assume a mechanical failure of the pump or other major problem:

(i) Check cooling fan is coming on - fan or its sensor could have failed.
(ii) Check radiator cap - a failing (hard or cracked) seal will stop system pressurising as it is suppossed to do and cause overheating. Best check is often replacement - they don't last forever.
(iii) In some car engines removing the thermostat can cause overheating - this is because it allows an accelerated flow rate through the radiator which gives insufficient time for the water to be cooled. So fit the new thermostat and check if that applies here.
(iv) Anti freeze is also an anti-boil fluid as it raises the boiling point of the coolant. If the mixture has become too weak due to periodic topping up with plain water (or from not changing it for many years) you can get over heating symptons.
(v) Similarly if the engine has been run for awhile without enough anti-freeze in the mixture - scale and corrosion in the passageways of the engine can be restricting flow. Descaling and flushing out engine and radiator with lots of clean water may then help.

If all of the above don't solve it then:
- could be failed water pump.
- could be a blown head gasket.
- could be an ignition timing issue ( but you'll usually notice lots of other symptons such as lack of power first).

Try the simple ones first - in my experience 90% of the time its one of those. Good luck.

__________________
Martin

finally back on the road again


http://awayonmybike.blogspot.com/