While the majority of out-of-tune hot rods on the street are running rich rather than lean, I recently had an occasion where I couldn't get my motor rich enough at idle. No matter how far I turned out the mixture screws, I could not get my 460 below an Air-Fuel Ratio of 14.8:1 at idle. I'm not much of a carburetor expert so when I had my Torino in at the shop, Mike at Mike's Auto Perfection
remedied the problem so that I'd have some idle mixture screw tunability with my Holley 670. I recently threw the smaller carburetor on knowing I'd need something milder than my BIGS 950 to pass an upcoming emissions check. On top of that, the price of fuel made the swap an attractive "de-modification". This smaller carburetor yielded the aforementioned idle lean condition in its "out-of-the-box" form. Here's what was done to correct the problem:
Mike opened up the secondary bowl and removed the metering block.
There are a pair of idle feed restrictors in each metering block. To cure the problem, Mike would modify the idle feed restrictors in just the secondary metering block. I've got them indicated here with the red circles.
Photos 3 & 4:
With a small drill bit set, Mike determines the size of the existing orifice by inserting progressively larger bits into the orifice until there is some resistance.
A pair of calipers confirm the orifice is .026"
In order to enrichen the idle mixture and to provide the idle mixture screws some range, the orifice would need to be enlarged. Keep in mind that if the idle feed restrictor is enlarged too much, it'll have to be removed and replaced. In that case, Mike increased the size by 7% to .028" being sure to blow out any shavings. It turned out that this was enough to dial in a 14.2:1 Air-Fuel Ratio at idle with the idle mixture screws turned out 2-1/2 turns.
Bonus Holley Tip:
Since we had the carb off, we cracked open the primary block too and swapped in a 6.5 power valve. Here's a simple trick to make sure the gasket seats squarely between the power valve and the metering block. Hold the metering block up, seat the gasket on the power valve's groove, and install it as shown. This ensures the gasket will be in perfect register every time.