The needle valve on the carb might be sticking, I had a similar problem with my 11B a few years ago. I gave the carb some gentle persuasion with a rubber hammer before using the priming lever and all was well. A can a of carb cleaner in the fuel tank took care of it permanently.
It’s a good idea to change the long lines in the floor stiffener if it’s never been done. I lost the brakes on my Light 15 a few years ago when the brake line burst. I pulled it out and it looked like Swiss cheese in several areas. It looked ever worse after putting it to a wire wheel. Scary!