I would say vacuum leak. You can build a simple vacuum leak tester at home for free (if you have the materials):
You take an milk jug, and drill a small hole in the cap. Then you insert a piece of vacuum hose into the hole, so that it goes into the jug about 1 inch. Then you put a lit cigarette in the end of the hose, inside the jug. Hook the other end of the hose to one of the vacuum ports on the intake manifold. Then close the choke or otherwise block the intake on the carb. Light the stogie and lightly squeeze the milk jug. The smoke will billow out any vacuum leaks.
Notes:
- Don't use "light" cigarettes, because the paper around the filter is perforated, which will make the smoke less dense.
- Having a helper search the engine with a bright light will help, so you don't have to squeeze the jug and search at the same time.
- Don't do this test too often, or the engine will start acting up every few hours or so, until its gets another puff of that wonderful stogie.
