'78 300 D automatic 223k mi. Just made major discovery: slow loss of power, clouds of black smoke. Adjusted valves,(were OK), did compression test: all between 320-350, rebuilt injectors, adjusted pop pressure to 2000 psi. Still ran like crap. I purchased tools for removal of PRECOMBUSTION CHAMBERS, and found all the holes around the bottom blocked 80% with hard, brown crud. Prechambers cleaned up, but I put in new ones anyway. Car runs like new! No smoke, will run 100 mph if you stand on it long enough. I searched everywhere on the internet for this, I hope someone else finds it useful.