Have 1200 hours on 912ULS, and always did mag check at 3000, with slightly rougher running on left ignition. Now have a very harsh vibration through the airframe when doing the same check. Recently changed carby on right side (#1 and #3) due significant wear in slide and body. Replaced spark plugs, did all ignition tests, can swap the problem to the other ignition switch by swapping plugs on ignition systems after the modules. Engine runs perfectly at all power settings, but harsh vibration also present when changing throttle settings between 2600 and 2800rpm, either powering up or down. Engine has two blade wooden prop, carbys balanced, tried replacement coil, no change, really getting frustrated! Took engine out and got Wal at Bert Flood to run on test bench, no problems present, but not sitting in the aircraft might not produce the problem. Did have kick back starting problems about 300 hours ago, got sprague clutch replaced and new flywheel mod to improve starts, starts and runs really well. Any suggestions??