I originally posted this yesterday on the your thread looking for a carb set. What you are describing sounds like something I had with my America associated with the carb float bowl vent, creating a vacuum in the float bowls and a fuel starving situation at speed and when there were wind gusts. I'll repost it here...
Your problem about a vacuum leak at speed got me thinking about an issue I had while travelling at speed some years ago. First off, there are 2 vent hoses that can cause problems with our bikes. The vent line for the fuel tank and one for the carb float bowls. The vent line for the fuel tank can cause your bike to quit if it gets kinked or driving in heavy rain. The fix was to cut the hose at a 45 degree angle to prevent water plugging the hose. I thought I did this to my fuel tank vent but instead I cut the carb float bowl vent just before a trip across Canada years ago. These hoses are just behind the engine and are easily mixed up.
My bike started to run really lean at highway speeds especially when trying to pass a transport or heading into a crosswind. I guess by shorting the wrong hose, it ended up in such a location that a vacuum was created in the hose/float bowl when the bike was at speed. I ended up with a loss of power along with real good gas mileage. I didn't solve the problem until I got to my destination. I couldn't figure out why my bike was running so lean at speed. I thought I had a major vacuum leak. Pulled the carbs, checked the hoses, carb mount rubbers, float levels, hidden filter, etc etc. Then I remembered the mod I did to the hose and after checking, realized that I had caused the hose to sit in some form of vacuum pocket. I relocated the hose and the problem was gone.
Food for thought before you change carbs unless you have confirmed the throttle shaft is leaking...
