Over the years these tanks develop pinholes. You can buy a gas tank repair kit that seals the inside with an epoxy like coating. I tried this when restoring our bird. It stopped the leak but a lot of work. I eventually just put in a new gas tank in and it looks much nicer than the old. The repair kit was given to me but think it cost about $30. A new tank runs about $100 (purchased a USA made one on ebay). Either option will require the tank to be removed. Not a hard project and only takes a couple hours.