Is the throttle opening fully? Plastic bushings (see link below) on the throttle linkage are known to fail which can prevent full throttle. Mark

http://catalog.peachparts.com/ShopByVehicle.epc?q=1986-Mercedes--Benz-560sec-Fuel--Injection&yearid=1986%40%401986&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6301%3AMBC%7C1546%3AED%7C10000060%40%40560SEC&catid=240621%40%40Fuel+Injection&subcatid=240696@@Throttle+Bushing&mode=PA