Age and quality of craftsmanship at time of production. Seems everything nowadays is disposable. Back in the day things were built to last. Nowadays Lincoln owes our shop more money for warranty repairs than I owe them for parts.
Once a 1980's or 1970's pipeliner is set perfect to factory specs from end to end they are smooth 285 amp machines. But if people have had their hands in them, changing things, thats where it starts changing. I have offered up to many a welder to come into our shop and weld with my red face and 76. Then I just sit back and listen as they say " I have always been able to tell the difference when running a red face, but not with these 2 ". So just realize Lincoln created specs on these machines for a reason. And once you start staggering brushes, rolling the rack, changing the idle and governor settings,etc you are changing what made these machines great. And then you are fighting a battle to find the sweet spot you once had. JMO