how can i fix my hob? Elextrolux?

As hob burns gas i can smell petrol like smell the flame is mostly blue but there seem to be a irregular yellowish coming through, i've kept the 4 hob burners running 4 a good hour as i thought i could've been air in the system but it's still the same, i've contacted the local gas people but as it happened a year ago already they've replaced it with a new hob charged a fortune and now we are back to square one again