That would usually be the sign of a vacuum leak..... But you checked everything, right? Don't really see where the power valve would cause a problem in the idle circuit either....No way there is a fitting missing or loose on the intake....? an unlit propane soldering tank is a real good way to check for vacuum leaks on the gaskets and carb base.... What does the idle do it you take the linkage rod off it completely? I've had a problem occasionally with engines with a lot of cam that I had to drill some secondary air bleeds in the butterflys.... Could also be a plugged vacuum port in the metering block of the carb possibly.... Which I was there to mess with it...got to be something simple....