4150 Holley swap for 1409 Edelbrock on 454 PCM

[Sdc77]

Hi everyone !
I finally did a swap of my old Holley (4160 with vaccum secondary and secondary metering plate). As it has a bad idle, with black smoke and unburnt fuel coming from the exhaust, sparkplugs are black, and mixture screws are tightened at maximum. Hesitation at acceleration as well.

The Edelbrock 1409 started good, just had to tune the idle screw to go to 800rpm.
I put a fuel pressure regulator with a gauge. Initial setup was 4.5 PSI (the factory mecanical fuel pump gives 6.5 PSI), power pump at 2nd hole.
I run at 300ft over sea level, use 90 aki octane gasoline with 10% ethanol (E95 E10 as we name it here).

The idle is ok but as I push the accelerator slowly, it hesitate, sometimes it stalls, and has no power until I go on secondaries. At 20mph it keeps the speed ok.
When I push accelerator quick, it backfires from the carburetor and stalls !
I tried the 3 holes on the pump : 1st it "paffffff" from the carb and stalls, 2nd it "paffff" and keep running if I go slowly, 3rd it stalls.
Went to 5.5 psi on the fuel regulator : no change.
Ignition setup is 8°BTDC @ idle 800rpm (as mentioned in the 454 PCM manual)

The floats are tuned the right way with 7/16 drill bit (I had to ask for new floats to the ebay dealer as the carburetor came with leaking used floats !), ah did I miss the fact that it's a remanufactured carburetor, actually a #9909 from National Carburetors.

It seems that the primary system is too lean for this engine, am I right ?

Is there a difference if the floats are set higher ?

Maybe the mettering rod spring is not strong enough for this displacement engine causing too lean mixture with the 454 intake vaccum ?

National Carburetors propose the kit#1488
On the Edelbrock forum, they propose to go to 10°BTDC

Do you guys have any recommendation for my setup ?

Regards