Need some more info....polarity(polarity is often over looked when changing from FCAW to GMAW), wire diameter, electrical stick out, sheilding gasses being used, materials being welded would be a good start.
To get a general place to start, take a quick peek at the welding wire manufacturer's recommended settings. Any wire mfg. worth their salt will publish these numbers for their product. If they have a website, look for product data sheets(PDS) or the like.
As a rule, with wire welding of any type:
Spatter = too much wire feed for the voltage setting.
Burn back into the tip = too little wire feed speed for the voltage setting.
There are some things to consider so that you don't chase your dials all over the place. ESO(electrical stickout) is measured from the work to the contact tip. This ESO dimension must be maintained at all times or your voltage will not stay constant. Wire welders are constant voltage machines which means that they will vary the current to maintain a certain voltage.
Spatter happens when the wire isn't melting off fast enough and it jams into the molten puddle and splashes droplets of metal all over the place. To cure this maintain a consistant ESO, and either add some voltage or turn down the WFS. Wire welding, particulary FCAW, gets a bad rep sometimes because people don't always understand what spatter is and why it happens.
Hope this helps.