my pals 07 rev 3 250 overheated yesterday under strange circumstances. any help on the causes will be appreciated. a small amount of white emulsion - ie spoonfull came out of one of the crankcase breathers. thought nothing of it as it has sat for a long time and presumed it would be a little condesated water and oil. soon after it started to knock in fuel starved type way. rad was not blocked, fan was working (that we can remember) and it should have never have got hot enough to overheat in that time. the outside of the cylinder has very slighty disclooured and it has got very hot in about 1 mile of unavoidable riding. it has not seized. it has not been stripped down yet to investigate. i think the water pupm may have stopped as even wth no fan on a cool day ridng on open fell with a clean rad it should have not got this hot. not yet checked to see if coolant has been lost into the oil. ideas or similar experiences anybody?

This might be a little obvious but you didn't actually say if you checked that there was coolant in the coolant system or not.

There is no crankcase breather on a two stroke. Do you mean gearbox breather or coolant system overflow line or mag case breather?

yes there was coolant in beforehand and i dont know whether its been checked since. i meant the the mag case breathers, the right side one i think. the speed at which it heated would make me think it was a circulation problem (or no coolant at all, which is easily checked obviously). do the water pumps pack up on these things at this age - how are they driven? the fan, coolant etc checks will be straight forward.maybe we will see the remaining oil is water contaminated, in which case which seal (waterpump?) is likely to have failed? thanks

Mine overheated even when fan working. I was laid in bed with the wife and suddenly thought its the pump thats packed up!!!!!

It's a little plastic impellar held on by a circlip.The drive part had worn circular so wasn't going round. Quite a cheap and easy fix if thats the problem.