Preheat oven to 350°; brush the inside of a 10-cup Bundt pan thoroughly with melted butter and dust it lightly with flour (use a pastry brush to help distribute the flour and tap out any excess).
Whisk the flour, baking powder, baking soda, and salt in a bowl until well combined.
In the bowl of a stand mixer fitted with paddle attachment, beat the butter with the sugars on medium speed for 2-3 minutes, until light and fluffy.
Add the egg yolks , one at a time, beating well after each addition, and then beat in the orange zest.
With the mixer on low, add the flour mixture (in three increments) alternating with the orange juice (in two increments), beginning and ending with flour and beating until well incorporated.
Stir in pecans.
In another mixer bowl and using the whisk attachment, beat the egg whites on medium speed until stiff peaks form ; fold gently into the batter.
Pour batter into prepared pan, smooth the top; bake for 60-70 minutes, until a cake tester comes out clean.
Meanwhile, make the syrup-in a small nonreactive saucepan over med-low heat, combine the sugar and orange juice, bring to a simmer; continue to simmer for 3-5 minutes, until the mixture has a light syrupy consistency.
Let the cake cool for about 15 minutes in the pan on a wire rack, then invert the cake onto the rack.
Brush the top and sides of the hot cake generously with the syrup and let it cool for at least 30 minutes more before serving.
Serve warm or at room temperature.
The cake will keep in an airtight container at room temperature for up to 2 days.