When the throttle is closed the RPM drops below the set idle speed and then stabilizes. If the idle is set where it sounds right the engine will die so  it needs to set a bit higher.  I opened up the carb and cleaned it out.  While I was in there I reduced the thickness of the needle washer.  The transient enrichment valve was a bit crudded up and wouldn't move freely so I cleaned that up.  It looked like there was fuel on the spring side of the enrichment diaphragm.  As I understand, it should be dry, or am I wrong.  I could not find a hole in the diaphragm.

Anyway, the engine is running very strong.  Almost all the backfiring is gone.  The washer mod made a big difference.  If I can solve the studder when the throttle is closed quickly I will be very happy.  Any ideas?

Idle speed is supposed to be 1100 - 1200 rpm. It is better to set it a little high than low. That will help reduce the afterfire (backfire) on throttle shutdown. It will help avoid engine stalling, and it will help keep adequate oil delivery up into the cam journals in the head.

Thanks for the sound file.  I have to idle a little faster but not much.  You helped put my mind at ease.