if the car shakes alot it´s most common not a problem of the idle, it´s a hint that you should take a closer look at your engine because there is something wrong with the air-fuel mix, or the ignition.
The idle was set before to a proper rpm. As long as the carburator or the throttle body wasn´t changed, the idle shouldn´t change.
I would take a closer look to the ignition system, a.e. spark-plugs, cables, rotator a.s.o., the fuel system, a.e. carburator or injection, filters, fuel lines, fuel pumps, injection black-box and at last at the air intake, sometimes its just a messed air filter.