My original reply above was 1/2 eaten by the server going down. Here is where I would start:

I think something is going on in the carb. Either it needs rebuilt, or for some reason the butterflys are hanging up and not getting to seat fully.

What do you know about this carb? Was it on the engine originally, and did it perform ok before the engine build?

Pull the carb off, and turn it over. Look down the venturis and see if the butterflys are closing tight. Back off the single adjustment screw on the throttle arm, and see if it will close more. You should see no gap in the bore to butterfly juncture. Sometimes the needles that adjust the idle are too far into the venturi bore, and stick out. Turn them out to see if this helps seat the butterflys.

What you are attempting to do is see why the carb thinks you want it to be in an advanced throttle position. If you have another carb, try it. See if it changes. Finally, put a new base gasket on the manifold, install the carb, and put a double return spring on the throttle arm ( one with a small one inside a bigger one........auto parts stuff) Start it up without the linkage on it, and see where it idles. (Oh, for starters, screw the needles in FINGER TIGHT, and back both out 1 to 1 1/4 turn)

Final thought, are you sure your tach is right? I guess you can tell if it is idling too high, but just asking.

If still no good, spray some starting fluid around the intake manifold and carb base while the engine is running. You are looking for vacuum leaks. If it speeds up, you have found the leak.

I still think you have a malfunctioning carb.