Well, what can happen are a few different things. First off, if your throttle position sensor isnt set right, you won't have the idle mixture anyway. It's important to make sure it is set right - a simple check is to remove the plug from the TPS at idle, the idle speed will change 99 out of 100 times if it's set right.

If your TPS is set right, if your air speed screw is too far open for example, your idle will be too fast end your engine too lean. But because it's got a low engine vacuum due to this, it will richen the mixture (because of the MAP's feedback) to compensate. If it's in too far, it'll make the mixture richer, but the heavy vacuum will keep it from being too rich - to a point. These two often cause lumpy idle under load where it might be smooth in park, for example. I've found idle to be one of the hardest things to set on my 4.5 PROPERLY to avoid lumpyness. I am resigned to the fact that it'll probably always be a bit lumpy with it set the way I want performance and economy wise.

I don't have my manual with me here (at work) so I can't tell you what dwell should be. I assume you know where the air screw is. Turn it in a half turn (or more) and see if your vacuum goes up (it should be higher by at least 3"). If not you definetly have a vacuum leak. The ECU's mixure screw is "Hidden" under the rubber boot on the ECU, on the side facing the radiator.