From past experience with my 84 500SEL ignition and fuel problems cause missfires which you can feel. If the engine runs smooth but lacks power, either there is a brake problem (not likely) or an exhaust problem. Cam timing could cause it but it would have to be from a 'gone bad' chain replacement. If you don't know the history of the car, you need to check the cam timing and the upper chain guides. If the guides are a dark brown color, they need to be replaced ASAP. If a chain guide brakes you will either spend a lot of time and money or junk the car. Usually most people WILL junk the car. That is how I got my car for $500.
How many miles are on the car?

Paul

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84 500 SEL (307,xxx miles)