If you think it's the gas tank, pull the line and run some gas into a container. Do you have a nice run of gas or is it choking along? Look at the gas in container. Do you see a lot of crud in it? You're tank's still dirty, so you're probably going to have to clean carb yet again and use tiny pieces of wire to clean all the holes. Get a filter on that line or this will continue.
Do you get it running at all to be able to ride it? Look up wiki for fourstroking and see if it sounds like it applies. Low end running is OK
When you have that carb out, check the jet number. It's really tiny and stamped towards the bottom. If it's an 85, it should still be running fine stock.