With the engine not running.
I looked a couple of my opposed twins full open throttle and full closed throttle.
full open throttle on the dash = (can feel the throttle lever shaft at the carb slightly forceful spring loaded to full open and some force required to force it too go to the idle position and will spring back full open when not held at idle.
As the throttle on the dash is moved from full open to full closed towards idle the carb shaft moves less than half way closed towards idle and is only slightly (very weak) spring loaded when at idle position.
Carb shaft just very slightly spring loaded. (not hardly any governor spring tension felt with throttle on dash at idle)
I suspect your throttle control plate or governor springs are not connected correctly or someone has installed wrong length connecting links.
You might have to remove the throttle plate from the engine and look behind at the springs or go to ebay and search opposed twin throttle plate and linkages, look at some pictures. I think maybe you could completely remove the little governor spring and your dash throttle would then without the engine running move towards idle. The holes in the governor spring linkages is too adjust the governor sensitivity so as the engine does not hunt and surge at governor speeds. (the stronger the spring force the more forceful the governor) The governor springs are also color coded per engine codes but I've never seen them very critical of operation by color when I'm making Frankenstein engines. (taking several opposed twins engines and throttle plates and making one)
