I have an old (maybe 20 years Old) Harbor Freight Mono-Mig 160 220V welder. It's set up for Argon/25% CO2 Gas. Over the years it's served me fairly well. Finally I got fed up with the inconsistent wire speed and decided to rebuild the entire welder. What I did:
1) Replaced the wire feed motor with a automotive worm gear electric window motor. I paid ~$25 for this. I got a few, and found one that I could control the speed down to practically zero with ample torque. The nylon gear train, crappy hobby motor, and worthless speed controller were trashed. I bought a DC motor PWM controller for about the same price that would work off of 24V DC. Next I trashed the 24V AC transformer, and used a Laptop PC notebook power supply that will work off of 220V input. Used the stock rollers and tension adjustment, but trashed the rest of the drive assembly. After this I have a super accurate speed control with a ton of torque that I'll match against any in the business.
2) The Mono Mig 160 has a decent sized transformer and also has a nice sized inductor. I installed a 76000 uFD 70V electrolytic capacitor between the + side of the rectifier, and the transformer to inductor connection. This really HELPS a LOT with stabilizing the Arc. I got this idea from a Hobart MIG schematic I found on the Web. A 50V capacitor should be enough.
3) I replaced the Bridge Diode assembly with one I made myself. Not necessary but I was having Fun.
4) I trashed the wimpy 220V fan, and installed a real 220V fan. Helps a lot with the duty cycle.
5) Trashed the Relay, and replaced with a 40 Amp Solid State Relay.
6) Added a 35 Amp Circuit breaker to the back.
A completely new machine that's a pleasure to use anytime. No more fussing with wire speed, the Arc is much more stable.