Yessir, this is first set of flights after annual during which we re-installed just overhauled statically-balanced MT Props. I'll ask the guys to do a dynamic balance. My "worry" is that they are or will be balanced to specs for cruise (ostensibly like they were before overhaul), but I've got something going on in the mags or injectors or leads or valves or mixture or something that is causing rough idle and gives the airplane the shakes at low RPMs. At 1000 RPM and higher, the engine runs more smoothly. My question is what to look at when they balance within limits but I still have "shaky" tail feathers and a tough time holding my beer while taxiing?
The engine is standard IO-360G overhauled at Mena about 2 yrs/100 hours ago. Since I got this bird last year there's always been some vibration, and as a career turbine pilot I'm trying to determine what "normal" vibration levels are for a 337.
All ideas are welcome at this point!
Thinking in public:
One other symptom (maybe?) -- the Rear engine prop control is MUCH more difficult to pull than the front. All mixture and throttle levers are about the same (easy), but the rear prop lever is orders of magnitude more difficult to pull to feather than the front prop lever. It works fine, just gives you more resistance when moving it back or forward. Guess I better ask the guys to check the cable routing for pinches.
I'm digging through the MT books, the TCM engine book, and the Cessna manuals trying to assemble a point-by-point logical order of things to check/verify for the mechanic$. They aren't sure what normal vibration levels are for 337s either. The Service Manual D2506-8-13 says in a note about idle adjustment: "...An engine should idle smoothly, without excessive vibration..." but we're not sure what "excessive" is.
We've verified a couple times that the correct prop SN is mounted to each end of the airplane. We've looked at the engine mount frame (no obvious cracks or looseness), the mounts (all appear good, no change in paint torque marks), cleaned, regapped and re-installed plugs (all ohm'd good, again).
I'll pull the spinner and spinner mounting plates & re-run the motors to see if that helps. I'd like to somehow eliminate the prop as out-of-whack so we can move on to the motor and start looking at idle mixture, fuel flow, injectors, leads, mags--and anything else you all can think of that might cause my booms & stabs to quake.