Capt. Bob is on the right track. The Ethanol is most likely causing your problem. It can be dissolved varnish. But was Oxygenated gas used in the tank? If so, it has MPBE in that gas. Which leaves a coating on the inside of the tank. As you use it, the more it builds up. And as long as you continue to use it. It doesn’t cause the problem, that sound like you are having. But once you switch to Ethanol. It dissolves the coating and it clogs carbs. and jets. It looks a lot like sand. The only way to get it out of your tank, is to have it steam cleaned or replace the tank.
