Look for a vacuum leak, cracked vacuum hoses, bad pcv valve etc. What do the plugs look like (assuming you do not have an O2 gauge installed)?

'64 Sport Fury, 528 Hemi, FiTech EFI, 4-speed, 4.10 Dana 60
'57 Belvedere 2dr sedan, current project in process
'19 Cherokee Trail Hawk Elite
'03 Ram 2500 CTD HO, 6-speed 214,000 miles and still going strong