Carbureted O-320 fixed pitch prop, Piper Cherokee.
Engine runs rough and vibrates the airframe substantially during power reduction from climb to cruise. Except for about 90% throttle, all other power settings are smooth. On a recent test flight. I slowly reduced power from full and then left the throttle at about 90% where the vibration were most severe. Engine data show some erratic EGTs on all 4 cylinders. I leave the throttle unchanged and start leaning the mixture, and everything smooths out again. With the mixture leaned, I run the throttle back to full and down to cruise again and never a vibration observed. The effect seems tied to a throttle position and not a specific RPM. What's happening??
The vibration is reproducible on the ground with full mixture. I haven't yet tried to go full power and lean on the ground because I'm a sea level flyer.
Here's some data to look at highlighted in the circle. There's a few Fuel flow blips from me playing around but the main story is the back half of the squiggly EGT and how they dramatically smooth out with a deliberate leaning input. Throttle position was not changed from the 90% location during this time.
Engine runs rough and vibrates the airframe substantially during power reduction from climb to cruise. Except for about 90% throttle, all other power settings are smooth. On a recent test flight. I slowly reduced power from full and then left the throttle at about 90% where the vibration were most severe. Engine data show some erratic EGTs on all 4 cylinders. I leave the throttle unchanged and start leaning the mixture, and everything smooths out again. With the mixture leaned, I run the throttle back to full and down to cruise again and never a vibration observed. The effect seems tied to a throttle position and not a specific RPM. What's happening??
The vibration is reproducible on the ground with full mixture. I haven't yet tried to go full power and lean on the ground because I'm a sea level flyer.
Here's some data to look at highlighted in the circle. There's a few Fuel flow blips from me playing around but the main story is the back half of the squiggly EGT and how they dramatically smooth out with a deliberate leaning input. Throttle position was not changed from the 90% location during this time.
