I have seen the same problem at our field. We determined the bearings are worn and the engine is over heating. If the bearings have just the slightest wear, ea. feel just the slightest rough or slightest play, they are causing friction witch causes heat and over heating you see the picture. richening the needle valve covers the the problem a little more fuel/oil through the bearing but as you know it doesn't work for long. The best way to check your bearings is to check them hot. Just after the engine is run rock them back and forth feel for the slightest roughness and hold your prop tips and rock to check for play. there should not be any. Just my 2 cents worth. I install bearings for the guys at our field.

Bruce