I bought a used 1995 Fantic Section 250 and the seller had little previous history of the bike. It starts easily, idles fine but when throttle is applied, it bogs down with no power. I thoroughly cleaned the carb and exhaust. I tried different float and needle levels. I am able to get it to run perfect for a short while. I turn off the gas valve and after about 40 seconds it runs perfect for a minute or two, then runs out of gas. It's a Dellorto PHBH26 CD carb with a 118 main jet. Any ideas what the problem is?