surging

[saf]

I have a frankenstein 5.7 gi carbureted (Edelbrock 1409) replacement engine in my Pursuit. The carburetor was replaced last year. I am getting a small surging at low rpm. The idle is set around 600 but it jumps around 30-50 rpm. Gets particularly noticeable when cold leading to engine dying sometimes. This small surging persists all the way to ~1200 rpm. Should I be concerned? How stable would you expect these engines to run? I tried looking for a vacuum leak spraying carb cleaning fluid everywhere - nothing. Cleaned the carburetor twice. The ignition is solid. My next step is to do a full carb rebuild. Any other suggestions?
P.S. Needless to say, tried idle mixture adjustments multiple times, cannot get the surging to go away.

 

[alldodge]

Dirty carb

 

[saf]

An update: I did a complete rebuild on the carburetor soaking it in chem-dip for two hours (did not look that dirty to me anyway, but I did it just to be sure) and blowing all wholes with a compressor. In the process I did notice that fast idle screw was missing - that would explain bad idle at cold start. Put everything back together - no change. I still get the same dreaded 30-50 rpm surging. Need fresh ideas, what else can I try? Ideas?...

 

[alldodge]

Surging is from a lean condition and the carb needs to be calibrated
See page 7 and cal chart on next page

https://www.edelbrock.com/pub/media/wysiwyg/documents/carb-owners-manual.pdf

 

[saf]

Another update: just happened to have Edelbrock calibration kit, so, tried going 2 steps richer in cruising mode. Replaced the rods. No difference, at least in idling mode, still getting the surging, did not try going full speed. I am really getting fed up with carburetors. Is it Edelbrock? I am considering going with EFI, did anybody have any luck with these kits?

 

[alldodge]

The Edelbrock is one of the easiest carbs to tune, and don't understand why yours is having trouble. Its either the carb (adjustment or dirty) or fuel pump (low pressure or restriction from antisiphon valve, collapsing fuel line, or other)

 

[saf]

The Edelbrock is one of the easiest carbs to tune...)

I know, this whole thing is driving me crazy. It "feels" like the carb is working fine. I can hear the hissing sound of air entering idle vent circuit, the sound changes as I turn the mixture adjustment screw and more (or less) air is being sucked in. I don't think the fuel pump is the problem. First off, I just replaced the pump and all filters last fall. Second, the engine is doing fine at WOT, the pump is supplying enough fuel @4800rpm to develop full power.
One thing that "feels" weird is that I have to back out the idle screw way out to get to 600 rpm idle, the throttle is almost completely closed to the point that even tiniest movement (as I push the throttle control against the idle screw) causes significant drop in rpms.
What about the intake manifold? Could it be the problem? I read that some manifolds are rated for higher rpms, e.g. 1400-6000 instead 600-4800. What if I have wrong manifold installed? How would I even tell?

 

[alldodge]

May have a vacuum leak, take a propane torch and barely turn it on enough to hear a slight hiss. Then move it around the intake and see if there is any change at idle