if you look underneath the grill the silver tube that comes from the gas knobs has an elongated hole in it, make sure this hole is clear by poking a length of thin fuse wire in it a few times. the yellow flame means there is not enough air getting to the burner with blackening of the burner. hope this helps
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix gas cookers.