The fact that the needle valve assy. was fastened to rear of the engine could
account for some vibration....but rather the leaky needle valve that produced
the bubbles, and some leaning out of the engine was the main problem. This
was cured in a lot of cases by mounting the n/v in a different location, or replacing
the leaky/bubbly one with another brand. K&B was a good replacement.

The needle valve was only part of the problems, which were exasperated by
factors induced by the engine owners.....fuel, props, mufflers, cowlings ect.

FBD.