Gearbox Cause of Vibration?

If you reduce throttle to fast you may be causing a momentary fuel supply issue at the carbs and it will vibrate, but it should go away like you stated.

If the carbs are synced well then go fly and see if slowing down throttle reduction makes a difference. If the carbs are synced, mag check is fairly equal and you have a lot of time on the gearbox and have idled too many times at low rpms then yes it could be a gearbox and it's time to have it inspected. Rule out the easy inexpensive stuff first.

Carbs out of sync at higher rpms is why I don't sync them at 2500. I found when I did that and then continued to advanced the throttle the needles would split again. So sync them at a higher rpm and the while sitting on the ground run the throttle on up and see if they stay together. Try the higher sync at 3500 and then see where that takes you.

Hi James,

To me your problem suggests sticking/uneven movement in your throttle control mechanism.

When I do a pneumatic carburettor balance,  I do it at  2500 - 3000 rpm and I check down to about 1800 rpm to highlight any discrepancies at Max Vacuum.

When I am satisfied that the carbs are balanced, I slowly increase power to Max Static / 5200 rpm (depending on propeller pitch setting) and then back down to fast idle 2200 rpm. I note any tendency for the carbs to go out of balance. Should a carb hesitate at some point or stay lower than the other, it is usually an indication of a sticking throttle system

The above is a test of  your throttle system movement,  which should be smooth and equal throughout the range from idle - full power.

NOTE: Above engine runs are conducted with the aircraft secured and with regard to prop blast.

Hi James,

Another thing to check is make sure both throttle cables move together in unison and one isn't slower than the other when decelerating.