Is vent tube different than tip over tube. The tube that hooks on to the right of the tank-correct?
I revved it up and popped a filter off. If that helps. When it backfired it came off. I'm also having an old motocross buddy come look at her on saturday. He knows a good bit about carbs etc. Used to race for Honda.

Blue/White, Battery Relocated, 155/45's, AI removed, Cocktail shakers