Frank, you have one heck of an engine here and it sould like you have checked it over pretty well. I think I know the cause of your problem. The idle stop screw is set too long.
My 75 had the idle screw set to long and the carb barrel wouldn't rotate closed enough. One word of caution here, use Locktite on this little screw or it can vibrate out. I bet if you adjust the idle limiting screw you will see this problem go away.