I just set up a custom domain for my Tumblr blog. I had high Google rankings and heavy traffic with my old url, and now I've lost my ranking and some of my traffic. I'm wondering how long I can expect my ranking to suffer? I'd rather not lose a big chunk of my traffic. Google has already indexed my custom domain, & I've verified it with Google Webmaster Tools. Tumblr automatically does the 301 redirect. Is there anything else I can do? Would going back to my old url make any difference?
How long has it been since the switch?
Have you confirmed the 301 is working correctly? i.e. does each page 301 redirect to their matching pages on your new domain, as opposed to just the home page.
Did you have webmaster tools set up on the old tumblr blog ? If so, did you inform Google the tumblr URL would be moving to a new domain? As explained here: Tell Google when your site moves.
If you didn't, than it might be best to undo the process briefly, install webmaster tools on the old tumblr URL and then change the domains again after, making sure to follow every step here: Moving your site