Take your tank off the bike. Put about 3-4cups of pea gravel in your tank with like a whole can of carb cleaner and another can of engine degreaser. I have used a little black sand blasting sand too. Shake the hell out of it numerous times. Keep shaking for 15-20 minutes. Your arms will get tired and you will have to stop and start again. This will be a good workout too. This will loosen all the crap up that is stuck to the inside. Empty all this stuff out and then rinse it thoughly with soap and water. Keep rinsing until ALL particles are out. Then blow it dry with an air hose or a hair dryer. Then take the petcock off and take it apart and clean it too. I have used this technique with 100% success on every cycle tank that needed a good cleaning including my Magnum. It won't work on step through models though. Unless your a gorilla and can shake the whole bike. If done correctly, your tank will sparkle inside. And, you will not have to keep cleaning the rust out of your petcock. The other thing you can do is install a clear in-line filter to monitor your success. It may seem a little radical but it works and it costs very little. Good Luck