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No, when you retard the timing the fuel does not get a chance to fully combust before it starts heading out the exhaust. You usually loose power and increase your hydrocarbon emisions.
Retarding the timing *can* reduce emissions under certain conditions, but it will definitely hurt both the performance and gas mileage. The *ONLY* reason to retard timing is to stop spark knock.
I think you got it backwards. Advancing the timing will give you more power. Your engine should already be 10 degrees btdc. I wouldn't advance it more than 13 degrees btdc. If you do, you will start to spark knock.