very common problem with these im afraid its the head thats warped and the air blows past the head gasket into the water jacket and compresses the cooling system i have had loads of the skimmed,i have also had to skim a barrel once before which is worth a though too.so take the head off and get it skimmed enough just to make it flat again,then buy a new head gasket(genuine one) and do it back up tight.if it persists which i doubt then you will need to skim the top of the barrel(take the studs out before of course).good luck with it ps take the thermostat out and put it in the bin too.
adam bike doctor
June 2010
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