Holley 350 rich as can be.

[rklein383]

If you advance the timing at idle and it picks up RPM, the engine wants the timing. Distributor will likely need to be altered to keep total timing at a reasonable level.

Main jets have zero to do with your idle if the engine is tuned properly.

Get the initial timing set properly FIRST! Then start tuning the carb idle settings.

Crackedback is right.... Main jets limit fuel delivery at WOT and have nothing to do with idle quality.

 

[nm9stheham]

Main jets have a bit to with idle; they feed the fuel into the bottom of the idle wells and the air bleeds feed in at the top of the idle wells; so a change in main jet may require a change in the idle mixture screws. But the final idle richness is set by the mixture screws regardless of main jet size; you don't change main jets to change idle richness.

1066' above sea level is not much altitude.