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I know this has been discussed a bit with minimal solutions. I think my situation is a little different, hopefully someone has a suggestion.

I have a RV-12, 912ULS, 600 hours. I have flown it the past year for 200 hours, it has run absolutely smooth for that time, Recently I am starting to notice a vibration that occurs between 2000 and 3000 RPM. It is suttle at the ends of the range and peaks at 2500. It is there from start up to landing whenever I am in that RPM range

The carbs are balanced, the prop was dynamically balanced, carb floats weigh 6.8 gm total in each carb. Cylinder head temps and EGT’s run very close on both sides. Any RPM outside of that range and it runs very smooth.

I found at 2500 RPM the left ignition switch drops 60 RPM and the right drops 150. I think this is a clue since the drop at any RPM outside the range is equal. I have replaced the right spark plugs (4) and that did not make any difference.

Anyone have suggestions?

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