We're welding carbon steel to EN19 type high strength low alloy steel using 308L flux cored wire, is this correct?
For welding carbon steel to an EN19 type high strength low alloy steel (AISI 4130 type), you should use a 309LMo type FCW and make sure that there is absolutely no source of moisture anywhere, so good quality gas shielding and clean base metals and excellent welding skill is needed to avoid oxygen pick up from the air.
If you are currently using 308 type flux cored wire, this is not suitable to resist dilution in site-welding conditions.
309 type will give very good toughness and strength, even with poor welding and variable dilution and low-grade base metals. If the EN19 component is thicker than say 10-12mm it should have a preheat of ~250C to guard against hydrogen cracking.