That can be indication of bigger problems like a blown head gasket but don't rush to that judgement. Was the reason for the fluid change just maintenance or were you having a problem? If it was just maintenance and the problem showed up after you changed the fluid, the following may help.
First, the radiator is supposed to be full, right to the neck, and NOT have any air in it which just aids in corrosion. That was the point of going to an overflow bottle on cooling systems. As the water warms it expands and all excess is pushed out to the overflow bottle. As it cools, it draws that back into the system, never allowing air to be in the cooling system.
Check the small diameter hose from the radiator neck to the overflow bottle for cracks or leaks. Especially where it connects to the radiator, the heat can tend to embrittle it making it prone to cracks and/or not sealing well to the spigot on the radiator. Also, check the cap itself. I usually replace the cap when I change fluid because they tend to get a ridge in the gasket where it has been seated and once disturbed, may not reseat properly. The purpose of the pressure cap is to raise the boiling point of the water so, if it doesn't seal, the water boils at 212 deg. F. (or less at higher altitudes). Bikes can run hotter than that in traffic and even as they approach that indicated coolant temp, localized hot spots may exist, such as in the vicinity of the exhaust ports, that can boil locally if the cap isn't maintaining pressure on the system. When the water boils, it expands tremendously and this will push the water out the cap to the overflow, symptoms similar to a blown head gasket. If you let it get too bad and the engine coolant gets low enough, it WILL blow the gasket or crack the head or do other real serious damage. Obviously, look for any other leaks. Hope that helps.
A note on buying pressure caps: Honda, for one, charges about $25.00 for a cap. Their neck is a very typical Japanese type that can be found on Mitsubishi and other brands of Japanese cars. I can't say for sure on other bike makes, but my last three speced in the manual to be a 13-16 PSI cap, and is the same as most later model Mitsubishi cars. Match yours up if you can because they're only about $5.00 at the parts store.
Keeping the "Hap" in "Happy Holidays"!
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Last edited by Dad; 10-24-2001 at 05:45 PM.