Just to make the conversation a little more interesting, I will play devils advocate here. Maybe the issue here isn't the company, but you. I personally find it hard to believe that you can't find "something better" to do at a startup. There are ALWAYS things to do at startups, and just because it's not within your normal scope of work, it doesn't mean you can't work on it. If you're completing your regular work, and you want to do something more (Ie. coding), and it benefits the company, I hardly doubt founders would be against it. What it sounds like to me, is that you are lacking any initiative and are diddling your thumbs waiting for tasks to be handed to you. If that's the case, you might be better off going to a corporate setting. Another thought that comes to mind, maybe you aren't given coding tasks because you haven't proven you are capable of doing them? If this is the case, then it's an opportunity to prove em wrong. Either way, the experiences you gain at a startup are what YOU make of it. Again, what it comes down to is initiative. Take it!

If you do decide to leave, find a job before you quit. Your negotiating power is much greater with a job in hand.

P.S I know nothing about you, so just take my comments for what it's worth.