Black soot ok plugs

If the choke is not hooked up, then to keep it running during the warm up period, you will have to either babysit it (lots of pedal pumping or a higher foot-controlled idle), or you will have to compensate it with rich mixture settings and or a higher float level setting, and/or additional timings. Any of these fuel band-aids can/will cause the soot in the pipe.The tighter valve lash may also cause issues, but not usually until the engine is fully warmed up. The loss of fast-idle will also drop out the vacuum advance, and the engine really needs that at this time, to promote a full burn of the fuel. Without it, the engine is running ,comparatively speaking, retarded, so not all of the fuel will get burned.
Also keep in mind that icing can occur at temps just a little lower than 43*F