In fact I tried a new carb wich I had available with a much bigger venturi (the original was 9.57 mm and this is 13.7mm). This is much harder to setup since in less than an 1/8 needle turn you can go from too rich to too lean.
Anyway, now I´m getting more power, like 400 or 500 more rpm and no quitting at all, even after many long test at very different needle settings, trying to find the best and as i said before, was a long and hard job.

Concerning the original carburetor I´m planning to buy a repair kit and replace the pump membrane and keep searching. I don´t think the engine turns too hot neither is a needle setting problem nor a popoff pressure, since the engine performs perfect at all ranges and transitions, for 3 , 6 or 9 minutes before turns crazy.