pull up bar

I have an older Century Quick Fix wire-feed welder (with a place for a gas connection), that I could use some advice for. Is there supposed to be a screw or something to hold the part that the gun wiring cable goes through that's on the tension piece? As it is, if there's even a slight "bump" in the welding wire, it pushes the whole assembly unit until it can't go any further then the wire simply starts binding up inside of the welder. There's a place where it looks like a screw or something used to go through (in) the tension unit then into a hole that's app 1/4" to 1/2" (if I recall correctly), away from the end of the assembly where it enters into the gun cable. Also, there's a small "plastic"(?) piece of tubing that the wire feeds through before it gets to the tension unit, but I don't think that's where the piece of tubing belongs. Any suggestion/ideas or even a description of where that piece of tubing actually belongs would help. However, I'm especially interested in finding out how to keep the cable assembly from moving. I've tried asking a representative but have yet to receive an answer.

I'm looking for a decent digital or dial caliper that is good quality but not so expensive. A quick online search shows calipers ranging from $20 up to almost $200. I like the idea of a dial caliper because there is no battery to replace.