Did you run a compression test to determine this? One possible solution may be to get a can of GM Top Engine Cleaner. It's a great carbon remover and frees stuck valves. Am sUre there are other products. I haven't tried this but have heard others say it works. My dad did it on his 740 w good results. Unplug a vacuum line that goes to the intake manifold. With the engine running, dip the vac line in the cleaner that will be drawn in by the suction or you can stick a funnel in the line if it's large enough and pour it in. Don't add much at a time or you'll stall the engine. Do it outside as the exhaust will smoke like crazy and have a very acrid smell. I'm sure there's a way to pour the stuff directly in the throttle body but it isn't easy as it's horozontal

Hope it works.

bl