Backfire when throttle closed

[flainn]

This weekend my GZ started backfiring when I let off the throttle. Pop-pop-pop. Never done that before.

Also, it does not seem to want to idle. I had to turn the idle screw way up just to get it to idle properly.

The only thing that I've done to it recently was, I bumped the muffler when I was backing it up. The problems seemed to develop afterward, but it could just be a coincidence.

Any ideas? I'm thinking it's a good time to give 'er a tuneup anyhow.