Set the oven rack in the lowest place in the oven, and preheat the oven to 350° F/175° C.
Place the potatoes whole in the pot, cover with water, and put on the lid. Bring to a boil, then reduce heat to medium, enough to keep a slow boil. Cook until the potatoes are very tender, about 15 to 20 minutes. Drain and cool.
In the small bowl, stir together the flour, cinnamon, allspice, cloves, baking powder, and salt. Peel the cooked potatoes and put them in the mixer bowl. Beat well, then add the eggs one at a time and beat well between each.
Add the brown sugar and beat until completely mixed in with the the sweet potatoes. Add the spices, sweetened condensed milk, melted butter, and lemon zest, mixing thoroughly.
Pour the custard into the prepared piecrust and bake in the preheated oven until the center of the pie is fairly firm and only wiggles slightly, about 40 to 50 minutes. Remove the pie from the oven and cool completely on a wire rack. Pie will keep in the refrigerator for up to 3 days, but is best eaten within 1 day.