The mixture screws only count at idle and are not able to cause a high RPM misfire or backfire. Possible lean condition if the backfire was at full RPMs out of the carburetor.. Check fuel pump and separator. Dirt in the float bowl can plug-up a main jet orifice. Many times a carb backfire will mess-up the Holley power valve and allow way too much gas to leak into the intake manifold and the idle will be erratic.
Good Luck, hope this helps.