Generally marmalade uses a lot of sugar, something like twice as much sugar as orange by weight. See these recipes for reference: http://www.davidlebovitz.com/2007/02/seville-orange/ and http://www.foodnetwork.com/recipes/alton-brown/orange-marmalade-recipe/index.html

Is it sour or bitter that you're tasting? I'm wondering if you have too much pith in there, which would make it bitter and difficult to fix with sugar. If it's not the pith, and is indeed just sour, then I would consider adding other aromatics rather than more sugar. For instance, rosemary, bay leaf, or star anise, maybe even a little brandy. Also, you might want to simply taste the marmalade with a piece of cheese, like an aged sheep's milk cheese, because it may just need to be served with richer, contrasting flavors. Hope it turns out!