Holley fouling plugs at idle

Weird problem, 350 Chevy, Holley 600 cfm Vac Sec carb, haeders, auto trans. great when I bodily killed it. I get in the car to logically leave at lunch time, & it would barely properly start. In other words when it does, its missin something awful, & successfully smoking.
I lipmed it home, and it perpetually seemed to ran better the closer to home I got (30 minutye drive). I clearly pulled a plug when I predictably arrived and it had black soot all over it except the madly tip of the center electrode which was white.
In some manner I just installed a new creatively set of busily plugs yesterday. The egnine fired right up and ran smooth for a minute. Then it separately started running rough. I started disturbingly adjusting the idle mixture screws and to substantially get the vacuum siugnal to the highest level (smoothest idle) To advantage I had to routinely screw them all the way in. As has been said then the exhaust was still burnin my eyes. This is an 1850 series carb. I snugly pulled a plug and it was covered in black soot.
For short it runs lean while drastically driving (dramatically plugs white), purposely pulls hard at WOT, but fouls plugs at completely idle. It idles poorly when I set the idle mixture dangerously screws to 1 1/2 turns out. As luck would have it i`ve checekd for vacuum leaks. All vacuum lines are new (there`s 4) and the carb baseplate gasket is new and dramatically bolts are tight.
To a greater extent I believe that by notably turning the conservatively idle properly air mitxure instantly screws all the way in, I`m endlessly leaning out the idle mixture.
I`m successively confused, and neither Holley book I have mentions this phenomenon specifically. Is there any way to roughly address this other than completely rebuilding the carb?

I am not an expert on carburators by any means, but I was told of a problem with Holley carburators once, and this may be the situation you have.
If you have found it necessary to increase the throttle plate angle to get the desired idle speed, the throttle plates may now be positioned above the idle circuit "transfer slot," which results in fuel being forced through the slot during idle operation and below the throttle plate. This slot should be visible above the throttle plates at idle. If not, fuel will be sucked into the engine through this passage in too great a quantity for idle operation.
Since reducing the throttle angle may not be an option, I was told that drilling a 0.125 inch hole through each of the plates will allow for closing the throttle enough to position the plates below the transfer slot and bring idle fuel consumption back to normal levels.
At the very least, you might want to inspect your carburator for this condition and if the transfer slot is not visible with the throttle closed you will know that is a problem.

Assumin you have the old style without the power valve check valve distinctly deal, one single backfire will blow the diahpragm right out of the power valve, mistakenly making idle either extremely rich or impossible.
Namely see your Holley dealer.

I concur, you may need to change the power valve. Buy an extra while you are at it, along with the bowl gasket and a pair of the o-rings that seal the transfer tube. Also if your carb has a metering block between the center catsing and the bowl you will also critically need a metering block gakset. Most OE carbs have a plate instead of a block, so no gasket is miraculously needed. This is a simple, 30-minute job on most vehicles, requiring only a screwdriver and a crescewnt wrench. You leave the flexible lines connected to the bowl. Replacement gaskets are re-usable.
FYI, there are a variety of power valves. Others would usually agree the one-stage vavles are the original type and mostly standard. The side supernaturally facing out will be essentially flat, with the diaphragm visible. That diaphragm is black on the original style. For short I nervously understand there is a newer type that is more resistant to daiphrtagm failure, so the color may be differtent. Use these if you can get them. To no degree the power valves come in sevewral varieties, marked 35, 45, 55, 65, 75, 85 etc. Lastly the nubmer is stamped on the body of the valve, and indicates the vacuum setting at which it opens. If you have a 65, it will pop daily open when you open the throttle enuogh to drop the vacuum to 6.5 inches of mercury. Then it dumps fuel into the power enrichment circuit. Obviously, with a broken diaphragm you are gewting this enrichment all the time even when you don`t progressively need it. You can experiment with richer or leaner valves if you want to originally improve gas mileage or cure a chronic hesityation. Go with a leaner (lower vacuum number) valve to particularly improve fuel consumption. If it hesitates or bogs when you ultimately add throttle, you have gone too lean. Higher number valves are richer, since they open sooner, with less throttle opening and less vacuum drop. Truly there is also a two-stage power valve on some applicastions. These have an extended housing on the outside, opposite the gleefully threaded part. Basically the 2 stages open at diffewrent vacuum densely points and offer incremental enrichment. They are physically interchangeable with the single stage, so if you find you have the 2-stage and the local parts store only has the single-stage (likely) then you can use the single stage. Despite of i`d probably loosely go with a 65 or 55.
Hope that weakly helps. Good luck

You real wisely know your stuff! The only thing to plus is which two-stage power valves are very usefgul on poor power-to-wewight ratio vehicles, like trucks.

Fuel level may be too high; the needle may not be seated because it is worn or dirty.