Hand piece has stopped working on my rossi mig 185.

SOURCE: mig welder tripping power supply

the breaker could be bad. try to change the breaker

SOURCE: MIG wire is always hot without pulling trigger

Check your manual, there are a lot of mig units on the market that are alway "Live" alternatively, the relay has either shorted or you have wirering problem on the switch.

SOURCE: changed polarity on the rossi 220 amp mig welder but still won't weld

What do you mean "it won't weld" ? Does the wire come out when you press the trigger? If so and there's no spark when you ground it then check the heavy relay that operates the heavy juice. Tell us what you have done and tried so far.

SOURCE: I have a Rossi 100 amp mig Gasless, has no grunt to weld with.

If you aren't used to mig welding it is possible you have higher expectations than the machine can fulfill or perhaps the machine isn't being used with the correct settings.

People new to mig welding tend to have the wire speed and hence the current too high for the output voltage. The wire speed control of small mig welders is generally very optimistic - my Clarke 130 is numbered 1 to 8 but I have never needed more than about 2.5 with the power on max.

If you are used to mig welding and are familiar with setting the power and wire speed so it produces the "frying" sound of efficient dip-transfer welding and you believe the machine hasn't the grunt a 100 amp welder should have, the machine is probably faulty.

There is one thing you could try and that is to reverse the polarity to a positive ground/negative torch. You can find more about this on Youtube.
Roughly speaking with the usual negative ground around one-third of the heat happens at the torch and two-thirds at the workpiece. It is the opinion of some people gasless welding is more successful when the situation is reversed.