<VV> RE: Main vs. Idle Jet

[grant young]

     There is not a main jet available for our carbs that is small enough to affect idle speed or quality. If, however, someone (such as a large aftermarket rebuilder like Holley, Carter, Rochester, or one of their sub shops) mixed the venturi clusters in your carbs once upon a time (or a PO simply swapped one of them) at some time in their life, it could definitely affect them. 
       There is a significant difference in idle jet size between clusters designed for all the different engines. But, with all the swaps that have taken place over the past 40+ years, the key is to make sure the idle emulsion tubes are the same size and the clusters are of the same design. It is also possible for the idle jets to change size on their own due to varnish build up or corrosion. 
     All other things being equal, if you notice that the difference in the adjustment of your mixture needles is significant (say one turn out vs. three on the other), it might indicate you have mixed clusters. I say "might" because there are also four different designs of mixture needles (in technical terms: very long, long, short, very short), and not all the bosses where they are installed are the same thickness. Hope this helps,
The Carbmeister

<Agree, all gas is fed to the well though the main jet however the flow 
should be so low at idle that the difference between a 50 and 52 should be 
irrelevant.>