The 0.8-1.0 wheel that comes with it is for gas wire with smooth groove. The correct wheel for flux wire has knurls/teeth for grip. Lowering the wire speed like you were doing is similar to increasing the heat. Faster the wire the cooler the weld will be.
U Groove Wheel= Aluminum and softer wires like Aluminum bronze and silicon bronze, K (knurled) Groove Wheel is for flux core, V Groove = hard wire like steel. I generally get a set of each in 0.8-1.0 for my welders.
