Re: how to adjust the air/fuel mixture on older carbies
Last things first. In rebuilding the carby did you check the float level? If that is wrong you can get plenty of black smoke at all engine speeds. Get that right first before you head off for the idle system.
As for the idle mixture, realise that in general turning the screw in weakens the mixture. As you've probably found, it is a tapered pin that mates with a corresponding taper in the carby body. I'd recomend that you wind it in carefully until you fell resistance, that is when the taper bottoms out. Then, wind it out say two turns. That should set the mixture reasonably close for idlinmg. Remeber too that this adjustment does not affect engine speeds above idle, where the main jet system comes into play.
If it runs, start to carefully wind out the screw (ie richen the mixture). The engine should either smooth out, or begin to run a bit rough and hunt. Then wind the screw in. You should go past the smooth spot, and the engine will start to falter. Now its getting lean. Wind the screw out again until you find the sweet spot. Then your mixture should be OK.
Oh - and make sure the timing is correct too.