HI guys!

I have a 1969 model 250/8 sedan w114 m114 with dual Zenith carburetors. I have been trying to get this car to RUN for some time now. (Novice!)

The car will almost start now, but stalls on every attempt because there is a BACKFIRE through the REAR carburetor.
Is this a carburetor adjustment? If so, which ONE?? OR is it the timing?

Help!!
~Christy