idle surge (74 450)

[rix116]

I have a 74 450 SEL. Finally have gotten it to run really good- replaced 2 injectors, advanced the timing (doesn't ping).
But there's one problem ( I can live with , but it isn't right)
When it idles it surges dramatically. Once before this was "cured"by retarding the timing alot, but then the car was almost undrivable(missing, sluggish etc.) I'm happy with it now except for the surging (does it cold and hot if it's in neutral)
Also turning the idle screw has virtually no effect.
Any thoughts? Like I say the car has plenty of power now
(don't think it's starving for fuel).Haven't played with fuel injection box on the right passenger compartment. Should I? (The car no longer has to pass emissions)