KX85 overheating

[meteor]

Just wanting some options to check that I mightn't have thought of...
Sons bike. Radiator is losing water, noticed it after saw steam from bottom of overflow hose after a long uphill. Filled it and was ok for rest of the ride. Was ok after next 1.5 hr race. Last race he pitted and I noticed it smelt hot, radiator was cold so no water, saw hose had come off head and the kips valve cover had started to melt. Seriously hot. Shut bike down, let cool, reattach hose, fill radiator, start up, see radiator fluid bubbling and notice leak in head gasket. I've taken head off, looks ok with no warp, gasket shows no obvious split. Radiator not leaking, hoses all seem ok. Water pump spins when kick start is turned and piston moves freely. I'm going to do the top end replace the gaskets, radiator and hoses but... I can't find the source of the fault. Why would pressure build up to let the water escape out the overflow? and why would the pressure blow the hose off the head? is there anything else I should have checked or do?

[F5 Dave]

. . . . fill radiator, start up, see radiator fluid bubbling and notice leak in head gasket. . . .

Pressurising the system. Leak in head gasket most likely if bubbles. Presumably you saw it outside as well. Sheesh! The only place there is that much pressure is the combustion chamber. But you might want to see if there o-ring seals if the water is routed through the cases, but not many modern systems do that any more.

Get someone to check/surface the head & barrel if required & I bet it is.

[glegge]

also make sure the radiator cap is releasing at the correct pressure, and of course that the thermostat (if equipt) is operating correctly.

[F5 Dave]

no thermostats on race bikes & rad cap is always the hopeful part, but not often the reason. Rad cap should only be called into play when overheating, not the reason for it if you see what I mean.

[scumdog]

Whewww..for a minute I thought you said 'XN85

As you wre...carry on....

[meteor]

...Presumably you saw it outside as well... you might want to see if there o-ring seals if the water is routed through the cases...Get someone to check/surface the head & barrel if required & I bet it is.

Thanks Dave, yes there was a head gasket leak at the front top side of gasket. There's no water in the oil so no blown seal from the impeller shaft. I've looked at head, no warp I can see. Done a light sand with 320 grit on a glass surface and can't see any flat spots. What troubles me is that the pressure build up blew the hose off the head.. possibly the weakest point and maybe not as tight as it should have been. One of those that there's nothing absolutely obvious that's wrong. Thinkin I'll just replace it all and start from scratch. Cheers

[glegge]

OK, well my bad for thinking it will be OK to assume it has a thermostat just because it's water cooled.
sorry if i could have put someone wrong there, i have to admit i've never seen one of these machines, but was just thinking of things that may cause the symptoms.
cheers

no thermostats on race bikes & rad cap is always the hopeful part, but not often the reason. Rad cap should only be called into play when overheating, not the reason for it if you see what I mean.

[F5 Dave]

So have you checked the barrel is flat as well? more mass so not so likely to warp, but still possible, or could be some debris on it. A flat fine oilstone can be useful, - but if it isn't flat it will be no good.

[Desmo guzzi]

OR maybe that hose was loose and was the original reason for loss of fluid..... engine overheats damaging head seal integrity , you top it up and from then on you have 2 faults , the rest you know . Wouldnt hurt to check the ignition timing too .

[meteor]

OR maybe that hose was loose and was the original reason for loss of fluid..... .

Yes, that's what I reckon. And when he did race, it got hot, pushed out fluid, pressure built up and blew it off and blew the head gasket . I'm lucky I got it just it time.

Had barrel and head checked, both good. I did the top end and replaced the radiator and hoses. First race yesterday (2hr 20) not a drop missing. OEM radiator in NZ $475, hoses $80... I got one from ASI performance (ebay ex China) for $135 landed and in 7 days. I know I should support the local market but that's quite a saving and they had good reviews on the web.

Thanks for the advice guys.