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.
What size engine is it and what type of transmission is it? Do you have the service engine light on? Have you tried to take it and get it checked on an engine analyzer?
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