Miller Millermatic 251 Owner's Manual page 43

Problem Probable Cause
Wire slipping in drive rolls.
Welding arc not stable.
Wrong size gun liner or contact tip.
Incorrect voltage setting for selected wire feed speed on
welding power source.
Loose connections at the gun weld cable or work cable. Check and tighten all connections.
Gun in poor shape or loose connection inside gun.
Remedy
Adjust pressure setting on wire feed rolls. Replace worn
drive rolls if necessary.
Match liner and contact tip to wire size and type.
Readjust welding parameters.
Repair or replace gun as necessary.
OM-1326 Page 39