If the engine runs brilliantly then it is not spraying fuel from under the head. Of that you can be certain now excess fuel on the outside of a running engine is extremely deceptive and almost at times impossible to track visually. Since you say it stops after warming up it could be from the carb throat during the starting process or more likely from the crankshaft behind the prop bushing if this engine is a plain bering engine.

I don,t know anything about Irvines but if it is a plain bering often these types will blow fuel from the crank if they are well worn. This is really no problem and if the engine runs fine that just tells me that engine is just getting good!

John