The passenger side is running rich, too much fuel. Check for vacuum leak along intake manifold. It could be on the driver side indicating engine is too rich and drivers side has a leak leaning it out on that side and keeping headers normal color. If carbureted check for plugged jet or leaking power valve. Check gasket under carb for leaks, check any intake fittings for leaks. It's not likely timing by itself because 1/2 the engine cannot be retarded while the other side be properly timed.