1973 Other BMW Models smoke problem

Engine Mechanical problem
1973 Other BMW Models 6 cyl Four Wheel Drive Manual change miles

hi
i bought a car. Thats worked good with out any problem.
i parked it for 1 week & did the carburetor work & ignition system.
after one week when I started the engine, car was run good, but when the engine went to normal operating temperature, started the smoking.
the smok is blue same as oil burning.
on cold temperature it doesn't smok.
i asked to my mechanic and he told me the problem is maybe the valve stem seal and they are damage.
could you please help me. Because different people told me different things.I need the true answer.
thanks & best regards