I use simple green, works good and safe on plastics from what I've seen. Cuts through fuel and oil just smells bad to me. Tank seems to be sealing fine, O-ring on the gas cap seals when pressed down. Doesn't look like it's leaking from there. I wondered if maybe from the head or the exhaust because the concentration seems to be on top of the exhaust pipe coming out the back of the engine. I'll check bolts and make sure they're all tightened down good. I'll try to clean it and look for any leaks. There's fuel residue all the way up to the wires running to the engine that go along the back of the fuel tank and also residue on the tank itself. It's mostly concentrated it seems like though from engine back. I'll just try to check and see if there's a leak.