struggles to start when warm/hot

i have a speedfight 2 100cc, it was a pain to start cold with my standar exhaust but ran and started fine when hot. Ive changed to a performance exhaust that blows alot and now its a pain to start when cold and will not start if hot or even slightly warm, its got a crappy k&n filter on it and no other mods apart from the performance exhaust. Any ideas? thanks cratch:

 

have u set the carb up right

 

yeah its setup right, not upjetted it and now put standard exhaust back on and same thing happens, also found out that the exhaust port on the head has started to crumble away :O still not sure whats up with it :/