I just put my engineer hat on and I am thinking about the gap from the end of the carb to the firewall. Turn on your vac and take the hose and starting 6 inches from the wall start moving it forward. You will hear the motor start speeding up. That is a result of a reduced voulume of air flowing into the hose do to the restriction to free air flow the wall is causing. The same thing happens with the carb. No to that introduce air flowing through the cowling by a hot engine heating it up on the wau to the carburator. We all know hot air is less dense than cold air. There may be some attention needed in this area. Or I could be full of it again. Just food for thought. I have seen this happen many times of everything from hot water heaters with dirty screens to my Kawasaki Voyager that fed air to the throttle body from the left side of the motor between the 2 very hot cylinder jugs to the throttle body with the air path having 2 90 degree corners. Bad design. Didn't take me long to fix that problem. Major difference in air flow feeding the air unrestricted directly from the right side on the engine where the throttle body is.