What causes a NH carburetor to idle too fast?
It's their nature to do so. I suggest you just learn to live with it. If you idle them down ,they don't run right at higher speeds.
Dean, have you checked your carb linkage? Set the set screw at about 1/8 of an inch, then check if it makes contact with the carb contact when it is fully closed, if not, adjust the rod by bending it till you get the desired results. Hope this helps. I had to do a lot of bending, in different directions to make mine work properly.
I've never had a problem with them until the throttle butterfly shaft gets worn enough to suck air, then they idle to fast.
Check the fit of the disk in the bore. So many are worn to where they don't shut off the mixture like they should from the main feed so it is not actually drawing fuel from the idle circuit. If the disk fits the bore correctly, the idle passages are open and the throttle shaft is tight you should be able to get it to idle down as far as any other T carb.