All you've said indicates a way too rich fuel injection problem. These SL mechanical fuel injections do have mixture adjustment screws, however, they also have a coolant temperature sensor and an air temperature sensor which can malfunction. Also, if your coolant thermostat is stuck open and causing your engine to run cold it may cause the injection to run rich. These or other factors can cause a too-rich mixture and plug fouling. It may be time to check out the 'Good MB Shops' section on this forum for a reliable Mercedes repair shop in your area. At the very least, I wouldn't make any injection adjustments without checking a comprehensive shop manual for this fuel system.