Vacuum is supplied via a small passage through the mounting gaskets and insulator block. They must be in good condition, this was a weak point on the old `lay downs`. That is a 40 year old saw, the carb diaphragms and gaskets are likely hard and brittle. Also replace the fuel line and filter.
FWIW, that is a heavy slow revving dinosaur. You are a better man than I if you can pack it around all day