Engine Won't Idle

[kempfer99ta]

I have a 421 SBC that was just built and I can't get to run under 2000 rpms. It has all forged internals a lunati cam with .480 lift and 230 degrees duration at .050, with 1.6 roller rockers. It has 9.8 to 1 compression vortec iron heads with the modification to accept more lift. The intake is an rpm air gap from edelbrock, and a 750 holley double pumper carb. The distributor is a DUI hei unit with 24 degrees advance. The car runs great at 2000 and up. The total timing is set at 35 degrees and is all in at 3000 rpm. At 2000 rpm there is 23 inches of vacuum. When I try to idle the engine down to normal 800-900 the engine cuts off just under 2000 rpm. I have tried 6.5, 4.5, and 3.5 power valves in the carb. If I set the idle speed screw to where the normal idle should be the engine just fires a few cylinders then quits. The reading on the vacuum guage when it is doing this is 10 inches of vacuum. I have tried the different power valves and adjusting the timing with no luck. Any help would be great. Thank you guys. I love the site

[leon phelps]

how do you know the car runs great over 2000 RPM's? It sounds good?

Have you connected a vacuum gauge to it when over 2000 RPM?

[usa1camaro]

Have you got any other Holleys laying around? I would swap one on that I knew worked to get rid of it as a possible problem. What are your mixture screws set at as you're trying to get the idle down? I will usually get the engine running, idle down until it starts to stumble, adjust mixture screws and keep going back and forth until I'm satisfied. For a baseline on the mixture screws I will usually take them all the way in and then back out two complete turns. You will most likely have to recheck timing when all is said and done but you're in the ballpark.
I keep a vacuum gauge on the car while I'm adjusting the carbs also.

[kempfer99ta]

I know it runs great over 2000 because it sounds great , no popping through the exhaust, revs freely with no hesitation. the vacuum guage reads 23 at 2000. The holley carb is brand new, Had a 650 speed demon with vacuum secondaries on it before and did the same thing. I have the idle mixture screws out 1.5 turns, have tried them at 2. The engine doesn't sputter when i go to idle it down it just quits.