Fantastic discussion! Glad I found it. I am having the exact problem with the 91FX motor I got second hand. I have tried many things from installing a bubbleless clunk to replacing all the parts in the high speed mixture assembly to different props and fuel and glow plugs. Bubbles after the high speed needle at lower than full power remain. I pulled the backplate and immersed the high speed valve in water and pulled a vacuum and applied pressure, no leaks whatsoever. What I have discovered is that this engine runs very hot. I have measured 330 F at the head stabilized at full power with an infrared device. That was even with an extra head seal and lower nitro fuel and a cooler plug. My conclusion is that the high speed mixture being attached to the engine overheats the fuel going through valve and causes vapor formation. At full throttle there is enough fuel flow to cool the housing enough to clear the vapor, but there is not enough fuel flow at lower flow to prevent vapor. The only fix I found is an externally mounted high speed needle assembly.