Can the needle valves be interchanged? I am under the impression they can.
The needle valve and nut can be swapped. This was recently demonstrated to me
One must use caution before one "just swap out parts" as the earlier version NH used both throttle & choke lever arms that are considerably longer than the '26 - '27 version. Switching up different parts will result in the throttle arm being an incorrect length to operate the throttle correctly - same with the choke wire going to the bell crank.
Steve, you beat me to it!
The needle valve on top was the only thing that I was interested in, not choke or throttle shafts.
Shouldn't be any problem using the through block throttle rod from the steering column to the carburetor. It could be a problem if you are using some sort of adaptor to the over the block throttle mechanism which was originally used for the vaporizer. I don't know whether an earlier than 22 carburetor can be used, but I have successfully swapped between a 22 and a 26 NH by changing the needle valve. It is possible you might need to put an s bend in the through block rod. If so make the s at the end near the steering column.