This should be in it's own post for future consideration. After reading Coolfogman's problem it exactly describes what I had. Relocate the float bowl vent hose which is behind the engine or install a foam plug over it.
Here's my hypothesis - there is a vacuum formed just behind the engine at speed that gets transferred to the top of the fuel in the float bowl through that hose. This vacuum prevents proper fuel flow inside the carbs. Usually you have atmospheric pressure pushing on the fuel in the carb just before the pilot/main/idle jets. If you end up with a vacuum in the bowl at speed, fuel transfer will be curtailed. It is quite rare but has happed in the past with a couple of members, myself included.
