Mercedes 240D won' shut off -adjust idle speed?

I recently bought a 1978 240D so I could run biodiesel. I heard all the warnings, so when it started to get slugish I changed both fuel filters. It runs much better now, and has more power, but when I turn off the ignition the engine won't stop running - even after I remove the key. I tried reducing the idle speed with the key-shaped knob on the dash, but that wasn't enough. Does anybody know if there's a way to change the idle speed under the hood, and how to get to it? Adam

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