With your HEI the above video is useless. Hook up your timing light and point it at the BB hole. Start the truck and rev the engine in short bursts, If the advance is working you will see the BB move significantly. My personal vote is for a complete carb rebuild of both carbs by a professional who will give you a warranty. Although for the $ you will pay for that you could probably go back to 1 carb. Cheap fix, disconnect the offending carb or remove it and put a block off plate over the hole.
