[Dominik]

CDOG,
If the throttle plates are just too wide open at idle (more than 3/4 of a turn) your mixture may also get overrich.

In the 70s they used a solenoid to hold the throttle plate open during idle and shut the TP closed at ignition off.

Ingnition for these engines was too retarded to keep them idling with a small throttle plate gap.

Also, your power valves may be incorrect for the used cam causing open power valves at idle = overrich.
As does an incorrect float level. Can you see little drops in the carb at idle? - Watch out for backfire while peering inside!

I also think the carb is a little too big, but that has not much to say at idle. I used a 850 for my 496.

Dominik