why does huck ignore jim's warning

I'm not exactly sure which point in the novel you mean. In part 2 Huck wonders about the dead man, though Jim warns it's bad luck. A while later, Huck decides to go ashore and to find out what's new. Jim agrees but with the whole snake episode (Jim was bitten by a snake's mate when Huck's prank goes wrong) makes Jim weary of bad luck. Jim insists Huck disguise himself as a girl, with one of the dresses they took from the houseboat.