I had lots of problems with both of my welders and suddenly realised why. Both fed the wire through very erratically, meaning that I couldn’t make consistent welds. Here’s my solution:
Basically the two posts were (being made of plastic) leaning apart a bit, meaning that if the wire was in its groove, the top roller missed it by bearing against the inside edge of the lower roller.
Bracing the two posts together as shown has made it feed consistently ever since. Sad really, as I think all SIP MIG welders have this failing, certainly my last one did.
Additionally, Machine Mart sell a metal replacement wire liner (the tube that the wire feeds through to the torch) which further improves performance, but I haven’t needed to change my plastic one yet. The Machine Mart part number is 010140118.