Ktm 450 sx wont start when hot.

Had this very same problem with a KX 85 last week! Turned out funnily enough to be the spark plug. So try this first mate. If that dosn't fix it then you have the age old mechanics question "did I do somthing wrong or is it just a coincidence". Start back at your beginning next & re-check your valves.Most of these late model bikes(12000-16000rpm screamers) have titanium COATED valves. Once the coating wears off they close up valves REALLY QUICKLY!!!!! If the clearances are ok your work was good & at least it will rule out insuficient valve clearance as the cause to your problem.If it still plays up - well then I would use the process of elimination to solve the problem i.e is it electrical,fuel, or mechanical. With valves checked already a compression or leakage test would pritty well rule out the mechanical system.To test electrics on motorcycles without test equiptment & a lotof know how can be difficult & EXTREMELY DANGEROUS( shock hazard 40,000 v+). So I'll try keep it simple. On the web you can purchase ignition spark testers that will connect to any plug lead & cause the ignition system to jump a gap of 6 to 8mm.These are cheap & will test the system as a whole for its performance. Great for a quick check of any ignition system. If the spark seems fine then your left with the fuel system. check for blocked fuel tap,water in tank or carby bowl,blocked fuel filler cap vent,blocked carby vent hoses,accellerator pump function.
I hope this has been somewhat helpfull.
I am happy to help with further questions
Regards Andrew