Gas fridge for boat

Question

Its switched for lpg. The burner wont stay on. The flame failure device has been changed and is working there is a spark . when i try it with the combustion chamber open or off it will light ok .its a sealed chamber about the size of a large match box with a air in take and air exaust out letits as if there is to much gas or air . i have tryed everthing to control the air encluding reducing pre airnation (thats spelt wrong) and my gas presure at the valve is 31 mil bar not that much above 28