The throttle doesn't have to be open more than idle speed to prime it provided the idle needle isn't set too lean and/or the idle stop set too low (idle gap too small). If an engine won't prime with the exhaust plugged and turning the engine over or blowing in the pressure line to the tank, there is probably congealed oil plugging the fuel inlet/needle assembly. Removing the carb from the engine and removing the needles from the carb and soaking in fuel will clear it out. Soak for a half hour or so. Blowing out with compressed air works well.