With flange-mounted carburetors that have lean-mixture problems, I've had leaking flange gasket problems nearly as often as gum-in-the-jet problems. This is especially so on Honda engines, which for a long time had insulator gaskets that stuck to both the insulator and the flanges, and split when the carburetor was removed. Any time you are working on a flange-mounted carburetor, check carefully whether it has been worked on previously. If the flange joint has been dismantled, there is a very good chance the gaskets will leak.
In this case Aldo, you know the carburetor has been worked on (replaced in fact) and you know there was some problem with reassembly, because the carburetor was jammed hard against the cooling air cowl. I think you need to go over the joints on both sides of the insulator carefully, and satisfy yourself that you have flat surfaces and new or as-new gaskets.
