Consumables are the cheapest way to a better weld. A contact tip with a worn bore gives a wandering arc, wire burnback and unstable metal transfer. With 1.0 mm solid wire in continuous work a tip lasts a dozen or so arc hours, with aluminium and flux cored wire much less.
Match the diameter to the wire. For 0.8 mm use a 0.8 tip, for 1.0 mm a 1.0 tip. For aluminium use a tip 0.2 mm larger than the wire, because soft wire swells with heat and jams in a tip cut to size. Above 300 A and for mechanised work choose copper chromium zirconium tips, which take heat better.
The diffuser splits the gas and carries current to the nozzle, so its condition affects the shield. A diffuser blocked with spatter gives porosity that you will not explain away with gas or parameters. Replace it every few contact tips and blow it out at every spool change.
The liner has to match the material. A steel spiral for solid steel wire, a PTFE or plastic liner for aluminium and for flux cored wire with a soft sheath. Cut it exactly to length: too short leaves a gap at the diffuser and birdnests the wire, too long it buckles inside the torch.
Give us the torch type or send a photo of the neck and nozzle and we will identify it and put together a set for a full quarter of work.