Take the carb apart and clean it thouroughly. Sounds like a partially blocked fuel system... possibly an air leak under the carb would aggrivate things, so when you put the carb back on, use a little high temp RTV sealant.

Another possibility is the ancient gasket at the backplate can start leaking. Neve hurts to clean off the old gasket and put in a new one (can be made from newspaper and red RTV... since that's going to be a hard to find part.)