Greg;
...to add to Derek's comment...we will presume Joe Curto et al, knowing their business, installed the correct late Throttle Plates, and correctly, but a well-lit view down the carbs, with Dashpots lifted to allow this, and some manipulation of the linkage would confirm they close precisely:
Source:
https://www.sw-em.com/SU_Carb_Thrott...ly_vs_Late.htm
...although it might be difficult to explain how these are affected by temperature...
...also..."...these carbs [are] (originally from a 140..." ...so if JC et al replaced them with the identical original Throttles, these carbs (may) have Throttle Overrun Valves (seen here with the characteristic solder blob):
Source:
https://www.sw-em.com/SU_Carb_Thrott..._Overrun_Valve
...and if one or both of these are dislodged, and/or sticking open (partially or more, including intermittently), which has been known to happen, it would certainly also result in an elevated idle...
I've even seen a Throttle with completely
missing TOV. leaving a gaping hole (and idle impossible to get below around 2000!)...it was presumably dislodged by a backfire, and later consumed by the engine...that continuous elevated idle was a trick to find (but obvious after the first glance at the holed Throttle!)...remedied by soldering closed the unwanted aperture.
Good Hunting!