Still has the standard camshaft? If so, it should not be idling lumpy, idle jet is too big [Idle jet also meters fuel for progression cct also]. jets are approx $6 - $10 online. Is the bridge still in the factory inlet manifold? the weber adaptor is mounted just on top. It is worth removing the bridge area to improve the flow characteristics. When you say backfire, do you mean the carb is coughing? unlike backfire out the exhaust. If so, it suggests a lean mixture condition is occuring, probably when the secondary throat throttle plate is opening, combined with no fuel.