Petcock flow is binary, it's either flowing or not. But doesn't really determine AFR, unless it's lean from flowing too little.
Rich could mean previous-owner hogged out jets.
Rich could mean float-valves aren't working
Rich could mean float-levels aren't adjusted properly.
These are best-case scenarios. Most likely you'll need
complete overhaul and restoration.
Also unscrewing mixture-screws makes it richer, not leaner. First find what the adjustment is set to before making changes. Turn in tightening direction and count number of turns and 1/16th-1/8th turns until it bottoms-out (i.e. 2+5/8th turns, 3+3/16th turns, etc). With that number, we can make a wild guess where to go next.