Originally Posted by mwardle7
Given that moving the jet needle clips only affects the throttle in the mid range, I'm wondering if it will make much difference at high altitude where I fly with the throttle wide open.
I think this is a misconception about the 912 carbs.
There is no connection between the throttle and the carb piston. The carb piston is moved by the difference between the venturi pressure and the pressure at the carb inlet.
This is influenced by the density and volume of air flowing through the carb.
So the carb is on the main jet at low altitude WOT. As the density decreases the airflow is less able to lift the piston even at WOT so the carb will transition to the needle jet. That is how the carbs get their altitude-compensating function.
I don't know at what actual power setting the transition occurs but it's not a throttle position. It must be related to (approximately) mass airflow. I don't know enough about the carb function to be sure but I think the piston probably moves proportionally to air density but the square of the airflow. The needle taper must therefore be a compromise, so the altitude compensation isn't perfect.
It would be interesting to rig a camera looking down the air filter to see the piston move with different altitudes and throttle settings.
In regards to the original question, it sounds like the auto fuel is starting to boil due to the reduced pressure at altitude.