I don't know anything about backfiring, but based on the diagram I would say that #56 is the mixture adjustment screw. I have a different 2bbl weber (32/34 DMTL) but it looks similar. The mixture screw on mine is recessed in its little hole and out of site; you may need to find the hole and feel around with a (slot-head, I think) screwdriver (the tool not the drink) .

Best Regards, Andrew
'63 SIIA 88"