I am interested in buying a car from a friend of a friend, they said it goes perfectly except the revs drop causing the car to cut out when you stop. What are the causes of this and underlying factors as I do not want to buy another debt. Any help would be appreciated.
In a car of this vintage there should be an idle air control valve. Especially if the car has been sitting for a while this valve could be sticky caused by buildup of oil or fuel vapors. It could possibly be removed and cleaned with carb cleaner or similar product.
A possibility is a vacuum leak somewhere. While running at high RPMs, a small leakage of air is not a problem, but when your RPMs drop, a small amount of air leans out the mix too much. The old way this was checked was to check all vacuum hoses for deterioration or looseness at the ends and if nothing is easily seen, try running at idle and spray the hoses one by one. You can use propane (potentially explosive) or carb spray, some use smoke. Your choice. Google the different methods and see what would be easiest for you. The basic idea for propane and carb spray is that when you hit the leaky hose, the idle will jump up as you are adding more burnable materials into the airstream.