I realize there is no impeller involved, but I generally do not run marine engines without water, or maybe I just did not see it in the video. Generally speaking, if the engine stumbles when the choke comes off it is a sign of lean condition, and I've found a good carb rebuild can clean the fuel passages out to restore the flow to correct levels. The idle circuit draws from the main, and the combination of varnish and lowered fuel level combine to run lean. Hope this helps., Michael.