Since the carburettor was taken apart it is possible that the vacuum lines are mixed up.
On the carburettor you can find three vacuum lines, the one towards the centre of the engine should be attached to the idle delay of the distributor, the top one on the other side to the advance and the bottom one to the idle control and fuel valve.

I would also check the ignition, bad wires and rotor+cap, or wrong or points setting can give similar effects.

Do not buy a new carburettor without knowing what is wrong with the old one. Swapping expensive parts is a quick and practical way of diagnosing a problem if you have then lying around or can take them temporary from an other car. Not if you have to buy them.
My first 110 had a completely worn carburettor from running for 600.000 km with GLP poured in from the top. It still ran fine on gas.

If the ignition is OK, the next step could be to take the carburettor apart. Since you don’t have much faith in you mechanic it may be a good idea to do it yourself. It is not that hard.