E21 318i (1981) engine tune-up

Ok, so i just bought a nice old bmw 318i off a friend for next to nothing and am now trying to give it a little bit of a tune up. So far i have replaced the spark plugs & leads, air filter & fuel filter. The car seems to be runing a lot nicer & smoother but it seems to idle at almost 1700rpm.

My question is: how do you adjust the idle speed on these old bmw's with the mechanical fuel injection?

Any help would be greatly appreciated.