I have a Lincoln 140 mig welder. It welds and cuts

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We have Mig Guns and spares to suit a wide range of Mig Welders. ... Torches are usually Euro Fitting and are typically fitted to professional mig welding machines of ... attitudes from people one is giving money to ! its great to get good service.

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Millermatic® 211 MIG Welder - MillerWelds

sounds like 4 hours of welding meets the welders 'Cycle time'

does the machine buzz a lot louder then usual if so you could have a diode gone on the welding rectifier if not check all connections even if the look good

Ask on weldingweb or miller welds, smokestack, they have a lot of good guys there.
Bryan

Sounds like a bad ground clamp or poor connection. The trigger switch may becoming intermittent too.

You may be exceding the duty cycle of your welder
Check your operstors manual to see how long you can weld at higher amps before giving the welder a chance to cool down

these need a good power supply , i looked and did not see the duty cycle on this welder if you are doing heavy welding 100% power it will need 30- 40 minutes to cool of to weld again. it all depends on the units duty cycle, stated on the welder spec plate ..can you bypass the auto feed speed mode ? this might be the issue.. good luck ..use all the protection you can when welding these are like the sun and can and will burn exposed skin and eyes .. if'n i was any help throw me a bone and vote thanks jay

1/4" plate with a 135 amp machine is probably beyond it's duty cycle abilites. If you are using a mixed gas, and pushing the max on the settings, it will be cutting out because it is designed to.
You might want to get a higher amperage welder for consistent welding of 1/4"
The 140 Pro maxes out at 5/16" using inner shield wire. and 1/8", or 10 guage with gas.

You could have a poor ground clamp connection. Check that. also increasing the gas flow will cause turbulence at the weld, this can have a negative effect on weld quality.
Check your connections, check your liner, actually, replace it if you haven't recently.

check main contactor or diode bank.