TL;DR

An inconsistent arc is almost always a worn contact tip, a damaged weld cable, or a poor work clamp connection. Inspect the contact tip first, it is the most common cause.

What you might see

arc breaks and re-strikes during a weld pass
rough crackling arc sound
weld bead irregular in width or height
arc wanders across the joint

Likely causes

Tap to mark as ruled out

Contact tip worn or bored out from prolonged use, causing erratic wire-to-tip contact
Work (ground) clamp corroded or loosely connected to the workpiece
Weld cable damaged, kinked, or poorly spliced introducing resistance
Electrode wire oxidized or contaminated with drawing compound residue

Required tools

Contact tip replacement set (match wire diameter)
Wire brush
Combination wrench for cable lugs
Nozzle wrench or pliers

Safety first

MIG output voltage is present at the contact tip whenever the trigger is in-circuit. Release the trigger and remove power before replacing the tip.
Welding arc produces UV radiation. Never strike an arc without a proper welding helmet in place.

Procedure

7 steps · ~20 min

1
Set the welder to a safe state: wire feed speed to zero and output off. Do not disconnect power entirely if the power supply is in a locked configuration.
Warning: The contact tip area of the MIG gun carries output voltage from the power supply. Keep the trigger released while inspecting the gun and remove power before replacing the contact tip.
2
Remove the nozzle and inspect the contact tip. A worn tip has an elongated or irregularly shaped bore rather than a round hole sized to the wire diameter. Replace worn tips.
3
Install a new contact tip of the correct size for the wire diameter in use. Match the tip bore to the wire diameter exactly per the Lincoln consumable chart.[1]
4
Inspect the work clamp jaw for corrosion or loose cable attachment. Clean the jaw contact surface with a wire brush and retighten the cable lug.
5
Verify the work clamp is attached directly to the workpiece or to a clean unpainted metal surface as close to the weld joint as practical.
6
Inspect the weld cable from the gun to the machine output terminal for kinks, cuts, or repaired sections with electrical tape. A cable with more than one significant splice in the weld circuit is a poor repair; replace it.
7
Make a short test weld on scrap metal. If the arc is stable, the repair was successful. If not, check the wire spool for oxidation: the wire should be bright copper-colored, not dull or discolored.

Sources

Lincoln Electric Power MIG Industrial MIG Welder Operator Manual, Lincoln Electric
Lincoln Electric Power MIG Operator's Manual, contact tip sizing and consumable replacement (general)
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