Thanks to Manny, Hit Man X, and David. After replacing air cleaner, running a can of 44x in 10 gallons of premium, replacing the spark plugs,
putting 40 lbs of air in the rear tires, and driving 125 miles on the expressway, it passed by 6 points--much better than last year's pass by one point.

The plugs were Champion...dunno who put those in! The deposits were built up, and the gaps wide.

The local Benz dealer had the right Bosch number H 9 DC non-resistor. I spent two hours looking on line to no success. Another hour calling local parts suppliers also brought no success...but when I called RBM of North Atlanta, they said, "Of course we have those!" The service manager drives a 90 300E and said, "That's the only plug to use." They cost $2.19 each.

Again, thanks!