Good News to all!!!!!!!!!!!!!!!
Found the problems!!!!!!!!!!!!!!
and the winner is................................................ ....................PROPELLER!!!!
I analysed the otto cycle graphic taking into account tah the cilinder head was really HOT. So I considered it as an energy loose on the graphic. This caused two occurrencies: 1- Excesive heat did not allowed a proper combustion (due to evaporation or compression loose or whatever) and 2- since the torque is directly related with pressure above the piston (and thus the muffler), with a small prop there were not enough pressure available for the tank.
As a conclusion to be taken into account: If the engine gets really hot (even if doesn't quit) a larger propeller is recommended to be used.
Anecdotically; I met a guy who had the same problem with a Supertigre 2300 (can you believe it? one of the most trustable engines ever built)... same simptoms than my GMS120; he also tried EVERYTHING in order to get an acceptable running and solution was as simple as: CHANGE THE PROP.
Well guys; I really appreciate your answers and I hope this thread served to anybody!!!! WORKED FOR ME!!!!!!
Best regards; thanks
Guille