How many miles on the car? The M117 engine are one of the best of the best but they do need some maintenance. Check the chain stretch and the color of the upper chain guides. If they are brown in color then they need to be changed. If they are white then they are new. The spring can be changed but make sure you have the cylinder with the broken spring at TDC so if something slips, the valve won't fall into the cylinder.
Do you know anything about the history of the car? Any work that was done to it in it's past life?

PaulM

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84 500 SEL (307,xxx miles)