Pro Systems 4150 750 CFM
U mean rich normal round town/ highway running....
1st that carb is way over sized for the rpms u are using.
I assume that u also have a very lumpy, high duration/ overlap cam... more designed for top end rpms...again if this is the case it is not designed to run efficiently under normal cruise conditions.
Timing.. If the cent and VA advance are in the ball park advance at the cruise rpms , not slightly retarded...it will give a poor burn.
A poor burn (wet plugs) doesnt always mean the carb is running rich....ANY of the above will cause the issue....and if big cam / big carb is the primary issue the real issue is a very old one... designing an engine with bragging rights/ ego in mine rather than designing for what the car will be used for.
A vaccuum leak will make the car run leaner.. more air less gas.
A poor spark due to bad plugs/ HT leads whatever will give a wet plug but will show lean with AFR.
Never close secondaries fully closed.. at least a hairs breath off, otherwise issues start, espec if not a daily driver of butterflies jambing in bores....bigger carbs do need secondaries closed up more for better idle
Idle mixture screws do not change to any significant degree mixtures once the car has moved off, butterflies opens up.
Intial timing (I asume not a EGR engine and therefore manifold vaccuum) is only for low load on starter and battery, easy start of the engine.. idle advance is made up of the intial+VA and should be in the 14 to 22 range depending on how wild the cam is... a good street 'rod cam usually in the 16 to 18 degree ball park.
It sounds like this engine has never been dialed in... the 'tuning specs' as one finds in the factory manuals, established.
If has a HEI then it will most proberly have to be converted in the VA and cent curves to a manifolsd vaccuum / non egr configuration.
This is all assuming float levels right, no crap in valves , jets etc...and carb has been dialed into ball park o2 AFR ball park numbers.
THEN and ONLY then can one even complentating setting up time curve specs