When trying to burn fat, what should my heart rate be?

I monitor my heart rate when i exercise. There's a fat-burn option on the stride machine I use and it tells me to keep my heart rate at about 127. However, because of this I feel like I'm not doing anything. Therefore I push myself to a heart rate of about 190. The machine always tells me to slow down to 127 but I'm not sure if I'm supposed to do less work than I want to do. Is this necessarily bad? Would it be better to burn fat by staying at 127 or should I continue what I'm doing?