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..

So did you leave the fan hard wired. Have you inspected the impellor as they do break being plastic.

have heard of similar problem turned out to be sheared flywheel key changing timing making bike over heat

Head O ring??

TLTEL

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?

Going back to the last 20 minute period that over heated, was the fan running the whole time? When the fan goes bad, sometimes it starts and sometimes it don`t.

By the way replacing the two head O-rings is quick and easy just to rule that out. De-carbon the head while your at it.

Replace the bike

Check to see if your fan is running slow. Mine overheats after 20 minutes and my fan is about half the speed of my buddy's brand new GG. If I connect a 12v battery the fan is fast, but on the bike it is slow.