camping weekend/coleman 2 burner

I took the family and some in-laws camping this past weekend at Interstate State Park in Minnesota. We had a wonderful time in spite of the rain - very few blood sucking insects were out. Since I had such a large group (11) I did most of the cooking on my coleman 2 burner. Friday night it worked perfectly. Saturday and Sunday it did not run quite as well - the flame was more yellow than normal and the heat output was inconsistent. Anybody have any ideas? Could it be a bad batch of petrol? Should I burn some carbeurator cleaner through the tank?

On another note: It is always fun to people watch at state parks as the groups are generally quite varied. I looked up Saturday evening to see a man who was old enough to know better squirting lighter fluid on his camp fire. Luckily he did not kill or severly burn himself or anyone else. I have to admit I have done this in the past, but I might add I was a teenager and alcohol possibly influenced my decision making at the time.