I have a 1978 puch maxi..Im pretty sure the carbs are the same though.
To richen/lean the mixture, check the left side of the bike (if you're sitting on it) for the carb. If you dont know what the carb looks like, just follow the fuel line. Then there will be a screw with a spring on it. Simply turn this screw in to make it leaner, out to make it richer.