Wrong gas/air mixture - might help to clean cooker burners especially build up of grease in air holes. This from www.gas-guide.org.uk/emergencies
"Although carbon monoxide is invisible and difficult to detect, there are ways that you can see whether an appliance may be dangerous. One is...
* An orange or yellow flame rather than a blue one
Carbon monoxide poisoning causes a number of accidental deaths every year when gas appliances are installed incorrectly, badly maintained or poorly ventilated. It is so deadly as you cannot see, smell it or taste it. Being exposed to carbon monoxide can cause brain damage and even death.
All appliances should be safety checked and maintained and it is especially important if you live in rented accommodation that the landlord checks and maintains any gas appliances. This is a legal requirement."
Peccavi
November 2008