A lot depends on the business. Are you starting off with a contract or taking a customer with you? You might be fine (don't forget that you've almost certainly underestimated your startup costs though).

One of the more successful restauranteurs I know, conversely, told me that the secret in his business is to plan to have no revenue for 12 months and then no income for another 12. If you can do that, you'll be fine for the long term as long as the food and experience isn't terrible. Even great restaurants can take a very long time to build a clientele though.

I would say though that - again, depending on the business - you'd be better off using your savings or going into debt than having a side job. Being able to focus everything on your new venture is a huge factor; also, knowing that you have to make those 12 sales calls because your income depends on it can really be a motivational kick in the right way.