Cars getting on a bit now so I am guessing that the carb needs a damn good clean out.
Take it to an old fashioned type of garage that specialises in carbed cars, there are still a few around. I had an old XR2 a few years back, sorry for that confession it ran like a dog when I bought it so I took it to a weber specialist, it came out 10 bhp to the good and ran like a dream, just needed a damn good clean thats all