I have just purchased a 1983 380SEL. It is a one owner car with the odometer showing 335,000. The car doesnt smoke or knock but it is missing on one or more cylinders.Ignition parts have been replaced. Are the valves adjustable( I can hear some valve noise)? Could the problem be a burned valve? What diagnostics should be done and is it really worth putting money into? Should the whole enging be rebuillt or maybe just a valve job? It is really a nice clean car.