My advice: Get 100% on your startup or don't jump the ship.
Don't fire bridges with your last job-contacts. Keep your friends and be polite with your CEO but tell him you have taken a decision.
Factor Number 1 in a startup is time, if you continue working for your job, you lose a lot of more time in between. You need to be in the position of having only two options: 1-success or 2...success. You need to be hungry for success. Take money from people you know how they are.