I have a 200 hour 912 . Flew to a remote air field a about an hour away plane flew great.. decided to head home rub up was great mag drop normal. As soon as I left runway engine started to shake terrible I was able to turn back to runway and as I got closer plus throttle back and engine smoothed out. I am able to recreate on the ground by running up to 5000rpm and after approx 10 - 20 seconds engine will drop about 1000 rpm and shake terrible. I tried switching to either mag and absolutely no difference. I tried turning fuel pumps on and switching to my reserve tank and fuel transfer pump still no change in engine shake. I pulled the choke and it would change sounds but shaking bad enough that it was hard to tell if it was better but at least it changed. I have tried checking carb diaphrams main jet is clear no debris in float bowl and floats are good. Fuel flow is great using electric boost pump and pressure never dropped when engine was shaking. I tested resistances on trigger coils and changed plugs which were all on the lean side but all very similar in color. Has k and n filters. Slides are free. I am at a loss as to what to check out do next. I have not tried new fuel but flew into aurora on a diluted version of that fuel(I have an aux tank so I filled it and pumped to wing tanks them flew them filled the tanks again when I left) anyone have any ideas?
