In a large, shallow pan, combine margarine, brown sugar, and white Karo syrup.

Cook over low heat, bringing the mixture to a boil and continuing to boil for 5 minutes, stirring occasionally.

Remove from heat and add baking soda, beating until well combined.

Pour the hot caramel mixture over the popped popcorn and pecans in a large bowl.

Stir thoroughly to coat all the popcorn and pecans evenly.

Spread the coated popcorn mixture in a single layer on a baking sheet.

Bake in a low oven (e.g., 250°F/120°C) for 45 minutes, stirring every 15 minutes to ensure even cooking and prevent burning.

Remove from the oven and let cool completely before breaking into pieces and serving.