Typically it's a sign that your LSN is too rich, but it depends on a lot of other factors. If you have the engine at idle and pinch off the fuel supply line, what does the engine do? Does it rev up instantly and then die? Does it take 2-4 seconds to rev up and die? Does it take longer than 5 seconds to rev up and die?

If you read the tuning bible, this is one of the first things that Ron Paris covers ;-) It's a great read and it stickied at the top of the forum.