Thank you for all the answers. Let me go by parts.
I actually fly on the top of a moutain 600 m abobe sea level. The final weight of the airplane is 3,5 kg. I'm now using a Master Airscrew because the landing strip is full o tiny stones that break off the tip. In my fisrt flights I used a APC prop and it went much better. I think that using an APC 14x6 or 14x7 will make a big difference, I'm sure of that. In my club the Master Airscrew are well know for their bending, but we normally use them becuase they usually get choped down.
Any way I'm still thinking the pump might be a good idea. I have revised the tank-carb alingement ant tried to reduce tha number of couplings, bends, and so on to the minimum. I think that is not the problem.
I still have the problem of the carburetor response. Any suggestions?