Could the water have leaked thru base gasket perhaps? I've reused metal gaskets numerous times without issue, do the zxr400 have somewhere else this could happen?

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You'll need a new gasket without a doubt, if you reuse the old one it will leak and you will have to strip it all down again.
If it's leaking coolant through the base gasket something is cracked.
Copper gaskets can be reused if you aneal them properly but multi later ones on modern engines are one use only!

Ok found my problem.. Previous owner who removed head accidentally unscrewed 3 of the 5 core plugs!!! So 3rd time round now and bike is perfect..

FYI reused gasket all the time.. So running sweet..

Can anyone send me some pics of the harness routing? So made some errors and have a serious air supply problem..

Thanks

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Worked out fine, previous owner unscrewed three core plugs in head, three failures later I found it out. Running good on old gasket :)

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Routing is on Chapter 1 on the ZXR400 manual, found on the ZXRWorld forum. Come on kid search bar is your friend
Page 11 Onwards, has everything to help with routing.
http://www.zxrworld.co.uk/Manuals/H%20M ... er%201.pdf
A tip I'd suggest told to me by a guy in the forces whose rebuilding his, is to remove the L shaped lube from the water pump. It's a single bolt. Part 132 on the microfiche and this allows you to remove part 32150. Seriously helped me when feeding the loom in. Obviously you need the coolant system drained.