The Main Burner does not seem to be burning correctly
Ensure adequate gas pressure to the appliance. The pressure can be checked by unscrewing the pressure test nipple and applying a suitable pressure gauge (i.e. a manometer). Ensure adequate volume of gas is being used. Once the fire is burning on maximum, turn off all other gas appliances in the house and calculate the fuel being burned from the gas meter.
Make sure that the burner is burning correctly. The flame should be even across the top of the burner before and coals are places on-top.