Sounds like you car is running to rich. You can turn it down using an allen key, think its 2.5mm. Their is a shaft right behind the air filter box. Put the allen key in their and do small turns to lean the car out. I adjusted mine to run rich and I went to far, mine did the same idle from 800-1000 until I turned it down some. As for the return spring, yes, their is. I had a shop replace the ball joints on my throttle and when they did they lost my return spring. It was on back order from Germany for literally 3 months. Needless to say I was upset, and I ordered a few extra springs so I wouldn't run into this problem again.
MBCA, PCA, BMWCCA & SCCA* member
86 190 2.3-16
83 944 ITS Racer