Lazy yellow flames indicate that not enough air is being entrained into the burner intake or the jet sizes are incorrect.
As you have observed 'lazy flames' create a lot of soot due to the incomplete combustion. The flames should be blue.
It is very dangerous to continue to use this cooker until a qualified gas engineer sorts out the problem. If you don't, you are going to have a lot of lethal carbon monoxide in your house.
Geoff in Wales, July 2010
