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By hard wired I take you mean bypassing the switch, I tried both and the fan and switch are fine and the impellor is perfect. Sheared flywheel key, I certainly hadn't thought of that, but wouldn't you notice a slight drop in performance from the bike? It starts and runs superb. Head O ring, this was the only thing I could think of but having no experience with O ring problems I was waiting to see if anyone would suggest it. Could this also result in a drop in performance or would it be negligible?

Originally overheated after a few minutes with the engine dying when the throttle was twisted. After a thorough check and clean of the fuel system, wiring, rectifiers, fan, radiator, thermoswitch and impeller, still no change so I replaced the thermoswitch in case it was dodgy. I tested it today and it ran superb for 20 minutes then it over heated and died with throttle again. I'm going to try a slightly richer mix of 1:60 instead of 1:70 but I am not convinced it will change anything. Any ideas? I'm getting a bit despondent..

Success!! The fan did come on TLTEL so I renewed the wiring as thats a five suggested. I also went round all the connections tightening them up and cleaned the earth. I can't be certain what was wrong as I went over everything again, but this time thanks to all the suggestions it ran like a sewing machine, it was pumping coolant and the fan kicked in and out. Fingers crossed for the field test! Thanks to everyone.

I also tested the fan with a battery and it worked ok.

Hi all, an update. There was no carbon around the the head, I changed the coolant and all looked clean inside and the pump is working fine. All electrical contacts appear clean and tidy and the plug rectifier looks mint. The only discrepancy was on the the two yellow wires for the fan rectifier, about 3/4 of an inch away from the rectifier on both yellow wires was a small burn mark where they may have been in contact with other wires. The rectifier itself is a Leonelli and it is clean. I went through the fuel system again thoroughly. I started on the second kick and while it was still cold it flew up the street perfectly. Unfortunately the fan did not start but I may have cured the surging. I tested the thermostat in a pan and it opened and closed perfectly. Does anyone have any suggestions before I do the refit?

Great advice, many thanks. It looks like I'm going to have to strip the bike and check all the possibilities that have been mentioned. It has to be one of them, or more!

Thanks Jon good point, I did suspect this as I may have put in some dodgy chainsaw oil/fuel by mistake as is was blowing a bit blue. I did attempt to take off the exhaust pipe but as usual the bolts were solid, being as you also suspect this I will have to get them out and inspect the head. Cheers..

It was perfect when I last used it but after standing for 12 months the bike is releasing boiling water from the radiator overflow and surges and dies when the throttle is opened up. I've cleaned the carb and jets, the reed valve is fine, new correct spark plug, the fan kicks in and off and the exhaust is good. It starts very easily and ticks over perfect but when I use the bike it surges and dies but will tick over fine. After testing it three times twice it emitted coolant and once it did not but still died when opening the throttle when it got warm. Any help is much appreciated!