first thing to do is look underneath at the gas tank if it looks deformed or flattened you have a blocked breather pipe this will need to be unblocked and then have the tank replaced. this is the most likely cause of your problemJamesVee said:Can anyone tell me as to why my gas tank will not let me fill past 12 gallons. It's a 23 gallon tank. I've had one station I visited where I was able to completely fill the gas tank. All other stations the tank fills at 12 gallons.