get the carb rebuilt with all the right new jets and parts that wear out.
I'd look at the atomiser and needle, a new fuel valve, smaller choke jet, go down a PJ size anyway due to age and wear, look at the filter under the banjo bolt, look at the slide and check the cutaway height is right for that model which from my chair is a /40 but may be more than likely a /50 (4mm or 5mm mouth).
I do these kits for 64GBP
regards
Taffy