What temp do you have it at? If it's colder, it'll hold the carb better. Otherwise, you may have a leak in your system. Fill a spray bottle with some soapy water and spray each connection on your system when you have the gas on, and see if any bubbles show up. If they do, you've got a leak. Other than that, I'm not sure why it wouldn't hold carb. Even water will carb after a little while.