Check that there are insulators on the injector mounts. There are supposed to be rwp phenolic washers around the attaching bolt on each housing -- one between the housing and the head, and one between the cap of the bolt and the housing.

These are often left off when changing the injector seals. The injectors then heat soak, and the fuel vaporizes in the injectors instead of spraying nicely into the airstream. It only happens at idle -- when running down the road, fuel goes through the injectors fast enough that it doesn't have a chance to vaporize.