UK Car Repair: backfiring MG, vacuum leak, jet orifice

Question
I have a 1977 MGB that is backfiring and runing rough ,I have replaced the plugs, wires ,distriber cap and roter button. I have also checked the compression all cynders are at or above spec. The car only has about 30000 miles on it. When I set the timing at 13 degrees btdc it shoots fire out the exaust about a foot if I retard the timing it seems to run better,but then wont rev up very fast any hep would be appreated thanks...

Answer
Usually it is worth checking simple things first; But in this case it is worth making sure you have reasonable compressions on all cylinders. If OK it could be starving for fuel, ie, too lean, or, getting to much air, ie, a large vacuum leak. Remove the float cover and check the level, it should be 7 1/16 measured on the lever in the cover, also look at the level in the bowl it should be just below the jet orifice. A vacuum leak is usually audible but check all of the fittings on the inlet manifold. Please let me know what you find.  Good Luck.