it sounds like your Idle ports are plugged and you have to adjust the Throttle blades open to the transfer slot's. I would check the base plate for some pluging. It might be a Vacuum leak as stated before. Spray some carb cleaner around the carb while the engine is running. Any change will be a leak. The HEI, set at 14 shouldn't be a problem. How old is the cam chain? Still sounds like carburation though. Good luck