I think your car has a knock sensor, and the timing may not be adjustable.
Saw that car for sale I think - is that picture you posted the one from the advert?
Idle/running problems are common to the 16v, plenty of people hear have gone through what you are currently/are about to. Worth searching on '16v idle' if you havent already
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black