Still sounds like you don't really know how to solder.. You probably weren't getting the wire and the clevis hot enough before applying solder. Also you need to clean the solder tip. When its cold you can use some steal wool. Use a wetted sponge when it's hot. You want to tint your solder iron tip with solder. Hold that on the surface to solder 10 sec or so. Then apply solder on the iron right next to the clevis and wire. Should flow right on. Remove iron. Soldering done. You might want to practic soldering with scrap wire. Almost any solder you can buy is rosin core. So don't worry about flux. You should clean oily surfaces but for the most part any surface that looks clean will solder. I have soldered a lot and the biggest problem is not heating the surfaces enough. When you have plastic insulation and other things to worry about it's a game between getting the wire hot enough and melting insulation.