man.. that's filter thing is freakin weird.. what could explain it except some obstruction inside the airbox? I'm stumped. Only 400miles? no way.
i want to tell you that you found the problem and make you happy so you jump up and down and go "YIPPEE" and kiss your old lady and .. well.. whatever :)
but i dunno.. wait and see. This ring carboning thing could just be the result of running rich lately.
I guess the exhaust port and header tube was pretty clear .. and i recall (?) you cleaning that muffler-tube a while back.
As far as why some extra air would "cure" a symptom of stuck rings, i really got no rational idea.
It might have something to do with combustion gas blowing by the rings, polluting the crankcase mixture.. and some extra air improves the mix ratio?
yeah.. all that transfer area makes a big difference .. the engine has potential.