Very hard to get a nice smooth idle.

Really? Both of mine are very smooth at idle.

Mine idles smoothly, but I've replaced most of the 25+ year old fuel injection components with new Bosch parts. Also the charcoal canister.

The charcoal canisters can make a mess of the system. If anyone is curious, look around on BenzWorld. There was a member who had an issue with one. It caused the fuel tank to completely collapse.

With every vacuum line, motor mounts, and replacing similar items I've never experienced anything other than a less than perfect idle. If an M103 isn't smooth, you're doing something wrong.