Just keep in mind that when trying to solve a backfiring problem stick to the pilot jet and pilot jet adjustment screw (or also called the air adjusting screw).  That is the primary cause of backfiring problems.   And is confined to idle to 1/4 throttle.
The white spacer affects the midrange of 1/4 to 3/4 throttle, where backfiring is typically not an issue.
When changing a pilot jet from one size to another, you should see a change of about one full turn on the adj screw setting.

Went for a ride. Adjusted along the way. Started at 1 turn out and backed all the way to half in 1/8th increments. And all the way up to 1 1/2 in 1/8th increments. The farther up I went, the back fire got worse. Got better the farther in I went but still there. When I got back, I turned it all the way in just to verify my setting. Guess what, the idle never slowed!! Checked the end of the muff, and a good bit of soot had accumulated. So.....I'm assuming this pilot is just too big.(57.5) I guess I'll put the 55 back in and give it another shot. Maybe it was boogered up or something since I had such a hard time getting it out.