I went on a low carb diet to drop half my body weight (315->165). Worked great, and I still keep my carbs relatively low (around 25% of my total calories). But on days when I ride (or the night before a race/big ride) I go ahead and eat carbs. Carbs are the fastest available energy, and as long as you don't go overboard (i.e., consume 1000 calories to fuel a 500 calorie ride), you'll burn the carbs while you ride.
Go ahead an have a yogurt, a piece of fruit, maybe a bowl of cereal or granola bar before you ride and add a carby snack every 45 minutes or an hour while you ride. If your total calories are less than your total expenditure for the day, you'll still lose weight in the long run, and you'll be able to take advantage of the boost in available energy.
FWIW, don't sweat it too much when you see you've gained a few lbs the day after a big ride. That's just your body stockpiling water and fuel to repair itself. Stay the course and your weight will continue its downward trend in the long run.
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Formerly fastest rider in the grupetto, currently slowest guy in the peloton
