If the throttle cable is too short or out of adjustment it could hold the throttle open enough to prevent the engine from starting.

1)With the throttle cable attached remove your carb and look through the outlet (the side that connects to the engine) and look at the small gap between the throttle slide and the carb body; there should be a small gap of a couple of mm.

2) If it is too big or small then give the throttle a bit of slack using the adjusters which should be located on the throttle cable near the carb and the handlebars

3) Adjust the idle speed screw located on the side of your carb to get the gap between the throttle slide and the carb body to roughly the right size

4) Fit the carb back to the cylinder head and attempt to start the engine, if it starts that give it some time to warm up then fine-tune the idle speed, if it does not start play around with the idle speed adjuster and try again

5) Adjust the slack in the throttle cable so that you have 2 - 6mm of freeplay on the throttle

If that does not work then you could try a new carb; the stock carb is a Mikuni VM22. You can get one from here http://www.ebay.co.uk/itm/MIKUNI-VM2...item35c187cedf

If none of that works the only other possibilities are lack of compression or an issue with the valve train. A compression issue could be caused by damaged piston rings, valves or a very bad head gasket leak. You could take it to a mechanic who will be able to do a compression test for you or you can by compression tester kits from Amazon for about £25.