You might say both. I work as a Power Train Product Designer (I design engine parts). After hours I work with a friend that has a cylinder head porting service we are dealers for Canfield and Brodex . Occasionally I build some real high end engines. Recently a 600 HP 70 Boss 302 currently working on a 351C that should come close to 700 HP if I can tweak the manifold to what I would like to have. Unfortunately I may be limited by the intake design.

Reed,

I have to differ with you on power valves opening at idle. If they do open because of vacuum fluctuations or leaking you can have some serious idle quality issues.

At idle... the fuel is metered where the throttle butterfly plate meets the bore of the carb. Tiny hole with a needle in it... on Holleys the screw is an air bleed adjustment. The power valve is on the main circuit .. where the fuel is drawn out of the fuel well...and through the auxiliary venturi, even if the power valve is open... there isn't any fuel moving through that circuit... the only thing that happens with a erratic vacuum signal during idle is you can hear the power valve clicking in the metering body, and this only happens if the valve setting is somewhere in the range of the vacuum signal.The only time fuel might move through the power valve during idle is when there is a hole in the rubber membrane. Then fuel is leaking on the other side of the throttle butterfly plates and running directly into the manifold.. and yes.. then you really have a problem. I don't happen to have a valve body out of a carb so I can't take a picture and show you...

You didnt mention the idle transfer slot. Idle fuel also comes through it. The tiny hole with the needle in it(known as the Curb Idle Discharge) regulates the amount of emulsion (mixture of fuel and air) from the idle fuel well and has nothing to do with air adjustment except in the case of the carbs with the reverse idle systems. At idle I have seen fuel being pulled from the booster venturis. Main jet size can also effect idle since all the circuits are connected in some way of course this would be an extreme jet change. I dont need to see pictures of carbs I have been modifying, swaping, cutting them up making 1 bbls out of 2 bbls, etc. since mid 60s. No offence ment but I have a 3D cut a way picture of a Holly burned in my brain and another picture would just cause me more nightmares of carbs chasing me down a long hall that never ends ..... LOL.

Funny... I've seen fuel coming from booster venturis at idle too... but only when the fuel level in the bowl is too high... or the idle is set so high that there is enough air flowing through the booster to lift the fuel emulsion from the well... Maybe we are talking apples and oranges here.......