Arthur , great idea to check throttle plate, Years ago a hack smog shop did that to Dad's 280SE, and failed to pass, he was in 70's then, the "fix" was to go to a nearby reputable shop (no names but) pay $2,300 for the MFI rebuild. Well, Dad called, and next weekend I went and undid the idle mixture screw, adjusted by ear and experience, and passed smog.
I will now use the cardboard test as a routine.