Steve
The recommended plugs are best only because they are non-resistor plugs. The resistors are in those big metal plug wire ends, about 1000 ohms of resistance, depending on model. If you start combining the resistor wire ends with resistor plugs, it's bad news. Usually these special plugs (platinum and split-fire, things of that nature) are actually resistor type plugs, meant to fit too many applications. A manufacturer like Bosch you would think would either make non-resistor plugs for applications requiring it, like MB's, or simply not have these vehicles listed in the application charts. But they're in business to make money, and they DID sell you a set of lugs, didn't they? Actually TWO sets, sounds like a win-win situation (for THEM anyways
).
On what you call the idle adjustment, you're way off base here.
That sets the mixture, not the idle. You really shouldn't be messing with that adjustment without monitoring the Lambda ratio (On-Off ratio) It's a critical adjustment. Yes, the part you mention is called the adjustment tower. If you need to adjust the mixture, you drill out the tamper-proof screws on the base and replace the adjustment tower with a new one. The new one comes with the new steel ball. You set the adjustment for mixture (which may very occasionally need to be done if certain work is performed on the engine or to adjust for engine wear), then the steel ball is hammered into the hole to prevent tampering. When you push down and engage the screw and turn, you are adjusting the top throttle plate, which is connected to the air flow sensor. Having this adjusted wrong can really mess things up later in the engine's life. I'd urge you to get it properly adjusted.
The idle speed itself is not adjustable, it's controlled by the idle control system, which is electronic. If the idle speed is incorrect, the idle control valve may be bad, or there is a problem with voltage supply (ie overvoltage protection relay), or possibly a vacuum leak "after" the control valve.
Gilly
PS I see Steve B has already commented on Steve F's previous post. I was addressing this to Steve Fraser.