HOW TO STOP MY GILERA RUNNER VX FROM OVER HEATING?

same problem with my gilera dna 70cc, it over heats after 5 mins of riding and when at 6000rpm i hear som strange noice coming out from the piston area.... i have burned one piston already :((( cost me £45 to replace it + had to take the engine appart to klean all the bits, also reneved the crank bearings and seals another £25... im not sure whats the problem why it over heats??? is it the main jet or is it my deresticted exhaust cousing it to heat up quick??? who have experianced it on their ped?? plz put some sugestions. thanks

donacebab
February 2009