As i said before the problem was the hi needle was set 1/4 turn too lean, after opening this 1/4 turn the engine runs just fine.
This engine is a stock Spe 26 and turns a menz prop 17x6 at 8000 rpm. The carb is a walbro wt-501 (9.57 mm venturi). I´m going to test a wt-542 carb ( 13.4 mm venturi) to check if i can get more rpm.
Thank you