Though I had made no recent changes under the hood, out of the blue, the 4160 was pig rich, wouldn't idle worth a hoot and so on. Confused, I unsuccessfully tinkered with various settings. Preparatory to hooking up a vacuum gauge, I removed the manifold vacuum port cap to find that it was wet. Lightbulb moment for this old fart. If the power valve itself is leaking, if the valve's gasket is leaking, if the valve is loose or the threading crossed, it's leaking. Leaking where? The only opening in the vacuum well behind the power valve is a small hole in the bottom of the well that connects to the manifold vacuum passage where I found the dampness. Voilą. And, yes, I did indeed discover that both the power valve (pic 1) and its well (pic 2) were wet. Long story short, a new PV was the fix. BTW, I never experienced a backfire or any other issue that might have caused the failure.
Anyway, if I'm incorrect on the particulars I'm sure one of the serious carb guys will chime in. Thanks in advance.
PVs used to be VERY prone to destruction from a backfire; sometimes one not even obnoxious enough to be noticed. They added "PV blowout protection" in their newer carbs, which is a little check ball in the passage that brings vacuum to the PV. Does your carb have that? If not it can be added easily.
Quality and age of the power valve makes a difference as well. I went through several valves from napa before finally buying a genuine holley. Carb was new enough to have pv protection allready. Whether the valves had sat on the shelf long enough for the diaphram to dry up or if it was just the quality of the part, I don't know.
