If I hit the throttle cut and hold it, or, accomplishing the same thing, pull the throttle trim back to zero, the idle drops a bit more, to around 3,000 RPM. I can kill it easily by putting my finger over the exhaust outlet (I'm using a 1/2" ID silicone extrension, the large Dubro one to direct the gunk away from the bottom of the plane, so no risk of burns).
What I'm thinking is that the throttle barrel is too loose (too much clearance built in by the machining tolerances) in the carb. I think the idea of an air leak at the front bearing and crankshaft (which was also suggested by the guys at flitelinesolutions) begs the question, "Where is the fuel coming from?"
The fuel is coming from the same needles and jets as when the throttle is open. On many carbs the barrel is slightly open on the bottom when closed on top, also there is sometimes a notch on the bottom. This is especially true with an air bleed carb. I do not know it you have an air bleed or twin needle carb. Also you are not getting near the the 15,000 RPM you should be getting form a 9-4 prop. I don't know how long your exhaust extension is, but have you tried it without it? It could be that excessive backpressure is causing poor performance and too much fuel pressure.