Prep. Preheat oven to 350°F with the rack in the center. Grease and flour two 9x5 inch loaf pans, or line them with parchment paper.
Mix wet ingredients. In a large mixing bowl, beat the canned pumpkin, sugar, and oil until well combined, using an electric hand mixer on medium speed.
Add eggs. Add eggs one at a time, beating until blended and scraping down the bowl as necessary.
Mix dry ingredients. In a separate mixing bowl, combine the flour, spices, baking soda, baking powder, and salt.
Add dry ingredients to batter. Gradually add the dry ingredients to the pumpkin-sugar mixture, mixing on low until just combined.
Add starter. Stir in the sourdough starter by hand, making sure everything is well incorporated.
Bake. Pour the batter evenly into the prepared loaf pans, smoothing the top. Bake in the preheated oven for 50-60 minutes, or until a toothpick inserted into the center comes out clean.
Cool. Cool in the pans for about 15 minutes, then transfer to a wire rack to cool completely. Cover with a clean tea towel after removing from the pans.