Ooh Larry, for a second there I thought we had you cold! Yep, the 280SL has the mechanical injection pump (6 elements, not 2 like the 230SL).
As for the backfiring, first start with the basics, plugs, points, dwell and timing. Make sure they are all AOK. Make sure the linkage is adjusted properly, and then before getting serious about the mixture make sure the fuel pressure and fuel volume is OK. There is a mixture adjustment but that is the last resort....