I *think* there is something you turn with an allen key in a whole down the middle of the air filter BUT I MAY BE THINKING OF SOMETHING COMPLETELY DIFFERENT so don't go on my advice! Can someone confirm this or correct me?

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190E's:
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black 2nd owner
http://www.maylane.demon.co.uk/190esmall.jpghttp://www.maylane.demon.co.uk/190esmall2.jpg