I just installed a 6Sigma jet kit in my 06 S40.  It's still backfiring, and I can't seem to get the mixture screw set right.

With the mixture screw fully seated, I have the highest rpm, turning it counterclockwise, the rpm's begin to drop at about 1 1/4 turns.

Loud backfire on deceleration, and a poof at shutdown

Pilot Jet 57.5

Main 150

added two metal spacers under the e-clip for a total of 3 metal spacers under it.

Altitude is about 600' here

Stock muffler has two 1/2' holes drilled in the end

poof at shut down and no adjustability, sounds like you should go to a 55

bang at shift is usually a header/muff air leak
you have 2 holes drilled in the end, not sure if that's the cause.

or you got your idle speed set to low
or your tev needs cleaning