What have you been doing to the carB? Why did you drill the butterflys and clip the springs? There are calibration kits available for these to allow for spring/rod changes without permanent changes. Rods and springs do not affect the idle circuit of these carbs.
The Edelbrock carbs are the easiest to tune, most reliable, most stable and best metering carbs on earth. I have used them all, Holleys, Demons etc. and you should be able to get the Edel going perfectly. And, no gas line on gaskets! No leaks, drips or runs. I have used these carbs with a wide variety of cams.
I have a XE274 and the best idle I can get is round 1000. That's a 234ish at 50 cam. So yours is in line.
The mixture screws are not working because you are running on primaries. When you drilled the butterflies, if that is putting holes in the primary butterflies, then the air bled thru the holes may be the problem.
Idles that do not snap back and kinda run down as opposed to return to there idle rpm are too lean in general. Generally, you have to get less than 1000rpms to work on the idle circuits of unmodified AFB carbs.
Another carb may work better for you, but I assure you that the AFBs will work as good as any other when tuned correctly.
Even tho I have not been too specific, I hope some of this turns gears for you.
Go to http://www.edelbrock.com/automotive/eps_intro.html
for more information on Edelbrocks if you do not have a complete manual.
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