In a Dutch oven over high heat, bring sweet potatoes, enough cold water to cover, and 1 tsp. salt to a boil, covered. Uncover and boil until potatoes are very soft, about 10 minutes. Drain in a colander.
Return potatoes to Dutch oven. Add syrup and butter, and salt and pepper to taste, and beat with an electric mixer, starting on low speed and increasing as necessary for desired consistency; for a smoother puree, blend potatoes in a food processor. (Whipped potatoes can be made several hours ahead; cover tightly and reheat in oven.) Transfer to a bowl and, if desired, top with pecans. Serve hot.