if you are seriously trying to bulk up then gaining some fat is a natural by-product of the calorie surplus needed to gain muscle mass. the real question is HOW much has your waist increased in relation to your overall b/w increase? if you honestly think you're gaining fat too fast, you need to cut some cals overall, not cut carbs and replace them with equal cals of protein and/or fats.
trim 200-300 cals (50-75 grams) per day off your carb intake for a few weeks, keep tabs on your results and adjust accordingly.

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