My carver gas heater makes a noise when lit?

When I light up my Carver 3000s gas heater, 1 the pilot light has a yellow look to it, 2 when I turn it up to heat it makes a quick spluttering noise and burns more yellow than blue, unless I turn it to really low it goes out. The only way I can use it is at very low, the flame then seems to burn 95% blue with only a little yellow. Could it be air in the system?.