Carlo,
Now this is turning into a real puzzler. It still sounds like a carb problem. It is a little troublesome to do but you can switch the carb positions. If the problem moves with the carb to the other cylinder, you will at least know the problem is the carb and not something else. You've probably done this already but you can check the carb boots for cracks or leaks. Also check the carb slides with the engine off. Have a friend advance the throttle slowly while you watch the carb slides to make sure they both start to move upward at the same time. You can also remove the carb bowls so as not to spill any fuel and see if they both have the same fuel level.
Don't know what else to suggest for now. Please let us know if any of this helps or not.
Bill.
