Doesn't take much to make your body burn fat. It's doing it all the time. Not giving it carbohydrates just forces it to only burn fat. If you want to ride a bike, you can burn either carbs or fat or a combination of both. After two hours of Zone 4 and 5 riding you won't be going very fast if you only rely on fat sources.
Carbohydrates will let you replenish your glycogen stores quicker than fats while you are expending energy at a high rate. So if you ride long and fast, you need carbs while on the bike. If you are leisurely riding and less than 1.5 hours, you don't have to have anything. Most of us that aren't anorexic by the BMI scale have plenty of fat for the body to use. Why add more?