The more cam overlap the more exhaust dilution at idle, the slower the fuel combustion speed. So the more need for early spark at idle, so the fuel burns quickly and on time.
Late idle timing makes you have to open the throttles more at idle
moving beyond the sweet spot with just the right amount of idel fuel transfer slot exposure at idle. The throttles must be in the this sweet spot at idle or you get a flat spot off idle.
On a holley carb the idle is shared by all 4 throttles.
Should be more or less even.
A Edelbrock and a Qjet idle on on the primary throttles.
Correct idle transition slot exposure is critical.