I have a 96 CB500 which has been spitting out coolant from the overflow / expansion chamber - at first when it got hot, but now pretty much whenever it's ridden.
So far it's been to three mechanics. The first topped up the coolant, and then decided it was a head gasket problem since it was bubbling and overflowing immediately.
The second said that the first mechanic had overfilled it, which is why it was spitting out the excess of the coolant, Also he said the thermostat switch was dead and replaced it. After that the fan started to come on once or twice and then didn't anymore. Yet coolant kept overflowing.
Then the official honda dealership bled the system saying there was an airlock, which meant the coolant didn't reach the thermostat to activate the fan. After that the fan comes on just fine, yet coolant still flows over. After taking it back to them showing them that coolant still gets thrown out (and more of it does when the engine is revved hard), they think it's a head gasket problem as well.
Some things to note:
1) Bike runs and sounds fine - plenty of power, no backfires, no coughing
2) Coolant is clean (not milky), the oil also is clean.
3) No unusual exhaust smoke.
4) The first mechanic did a compression test - one cylinder has 175psi, the other 125. He felt that was sure proof of a head gasket problem. The second mechanic said with a 13 year old bike you can't expect perfect results and if there was a problem with one cylinder, he would expect performance to suffer (which it doesnt).
What could it be? Head gasket blown, even though the bike doesn't show most of the symptoms?? Since all three mechanics have quoted vastly different amounts for changing the head gasket (ranging from £300 to £1000), I'm now not sure what to do...