I just rebuild a 292.  I cannot get it to idle lower.  I am pretty sure I do not have a vaccuum leak.  Any other thoughts?  This is a holley 600cfm new carb

If you go to the Holly web site they explain how to set the throttle plate for the suggested maximum setting.If you turn the carb over and look at the throttle bore you will see the transition slots.The throttle plate should be set so that you see only a "square box" showing under the throttle plate.Any more of the slot exposed and you are into the transition circuit of the carb and you will never get it to idle properly.If you go to the Holly site they tell you the  maximum amount of slot to be exposed which can be measured with a feeler gauge or eyeball it so all you see is a square.You dont say if the motor is stock or are you using a performance camshaft.If you cant get enough air into the carb with the throttle plates set to maximum slot exposure you can open the secondary throttle plate a small amount.There is either a screw to adjust in the base or a threaded hole where you can ad an allen key set screw to adjust the opening enough that more air is allowed in.Some carb "experts" drill a small hole in the leading edge of the primary throttle blades to allow more air but you must be careful not to go too big.The secondary adjustment is a better idea because you can control the amount of air allowed in.Just a suggestion from experience if you are sure you don't have a vacuum leak.You can check for a leak by spraying carb cleaner around the carb base and intake manifold with the motor idling as low as possible.If the RPM increases during the testing you have avacuum leak drawing the carb cleaner into the intake.Some people use a propane torch unlit with the valve open for this test but I would use carb cleaner. Just reread your post.There have been a couple of instances where new carbs(Holly) had loose base to carb body screws from the factory.You might check this also.

  

Just took the head off because of a leak and notice the carb was not tighten down.  Could be the cause.  Will know more when I put it back together.