I've been struggling lately trying to get those "light-tight" joints that are essential to good glue-ups. I'm not sure what I'm doing wrong, whether it is setup, or technique, but I'm getting 7 to 9 mil gaps on my joints from the jointer. It's a good jointer, a pre-international General, so I can't blame the tool, I just seem to have lost the recipe. My suspicion is that it is technique in maintaining good 90 degree contact with the fence, particularly for narrower boards.
Yesterday, after multiple setup evaluations and trying different techniques, I decided to take a different approach: using the jointer to get the edges close to flat and ready for jointing, then use a hand-plane to joint the two boards (folded in book-matched configuration) together. Wow! Light-tight joints! I'm a happy camper. Doesn't add that much work and results in a much better end product. I did determine that you can't get too aggressive, or the ends can wind up with gaps or the boards get re-shaped to the point of requiring another trip across the jointer to get back onto an approximation to flat.
I used my LN #4 for this. Several years ago, I got a #5 1/2 Stanley from Pat Leach; I need to sharpen the #5 1/2 blade and try it, I suspect the longer sole will provide a better reference.
Downside to this is that now I've got to start thinking about getting a jointer plane. Even if I do figure out what is going wrong on the jointer, I believe that the ability to match the two boards to fine-tune the joint will continue to provide superior joints for glue-up.