Excessive vibration

[Learjetter]

Searched, found nothing on point. Flew last week and with RPM about 2300, reduced throttles to 12-15” in the pattern. Felt an unusual vibration and subsequent flights with another pilot makes us think it was the rear engine vibrating more than I remember. We removed and cleaned and reinstalled all plugs, especially since we could hold the plugs cyl #2 on rear engine and the rest were too hot to hold. Maybe a little less vibration at idle on subsequent engine run. Rear engine has about 110 hrs since MOH at Mena.

So, how much tail boom and vertical stab vibration is “normal”? Had A&P listen to the engine runs and he commented on the amount of tail vibration he saw. Also, the rear engine can be moved 1/2”-1” in every direction by pushing up or down on the spinner. Is that normal for 337s?

What’s different since previous less shaky flights? Overhauled and statically balanced MT props.

I’m beginning to think the R mag doesn’t work well at idle. Both engines seem to run smooth and throaty for takeoff and cruise. But rear gets rougher and shakier at lower RPM on R mag only (about 150 rpm drop on mag check, about 100 rpm drop on L mag check)

I’d really appreciate any thoughts on next steps to try. I do not have a reliable engine monitor yet...but plannng to install EDM-960 or MVP-50s this spring.

[JAG]

Just to clarify - this is the first flights after overhauling and static balancing the props? If I have that right, the first thing I would do is have the props dynamically balanced. I installed new props this past year, and even though they were statically balanced and I had installed new engine mounts, the dynamic balancing took us a while to get the rear prop to acceptable limits. I have worked on a planes with props (cessna 150 to Lockheed Hercules), and static balancing is only for the prop uninstalled, dynamic balancing is for the prop installed on the engine.
Jeff

[Learjetter]

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.

[mshac]

I do find it very odd that your bird has two new three-blade props, a fresh rear engine, but no engine analyzer? Priorities, my friend. You could determine your problem almost instantly if you had one installed.

IMHO this has nothing to do with the props.

Cold plug from #2 cylinder should be your main clue as to what is actually happening.

The following steps are recommended:

1. Compression test for #2 cylinder - If it checks out OK, go to step 2

2. Clean & inspect fuel injector. DO NOT STICK ANYTHING INTO IT! You may want to swap injectors from another cylinder just to see if the problem follows the injector.

3. So by now you've verified fuel and compression. The last variable is of course spark. From your report, the right mag may be weak. You don't say if they were OH'ed with the engine. Likely time to send them both out for OH.

Following these steps will resolve your issue.

As an aside, one of my V8 work trucks has a bad coil, so only 7 of 8 cylinders make power. Of course, there's roughness from the missing power stroke, but also from the fuel that was supposed to burn in the cylinder exploding in the exhaust pipe instead. Point being a cold cylinder will cause roughness in more ways than one.

[Rick Erwin]

I agree with mshac in that your best clue to the vibration is the cool #2 cylinder. Figure out why there's no fire in that cylinder.

Your 150-rpm mag drop is right at the limit for my TSIO-360CB. Yours might be different... Perhaps once you solve the cool cylinder issue your mag drop issue will be better also.

As for the increased force to move the rear prop lever ... If you have prop-sync, that's most likely due to the rear prop lever moves the prop-sync controller which is on the rear-engine. On the front engine it's a direct linkage to the prop-governor ... as I understand it. I had the same question when I got my plane and that's what I was told.

Rick

[mshac]

Another thought for you -

Use one of those cheap IR temp pistols to check the cylinders and exhaust pipes. You know the kind, they usually have a red laser pointer on them. Maybe $25, most likely less. We'll call it the Poor Man's Engine Analyzer!

You can take some live readings while the engine is running. More than any particular temp, you're looking for DIFFERENCES between the cylinders.

Measure cylinder #2 (the suspect) with both mags AND L/R ONLY! You want three sets of data. Knowing what happens on L/R mag only is very helpful in diagnosis.

Measure the exhaust temp about 1" below the cylinder head. Measure the spark plugs, measure the heads. You'll quickly notice any cylinder that's "out of whack", and you'll have a good basis for determining the cause.

EDIT: Here is one for under $25:
Etekcity Lasergrip 1080 Non-Contact Digital Laser Infrared Thermometer Temperature Gun -58℉~1022℉ (-50℃～550℃), Yellow and Black
by Amazon.com
Learn more: https://smile.amazon.com/dp/B00DMI63..._kmq7FbZP5ZR8C

[Learjetter]

Thanks guys! I’ll do as suggested.

Mshac— the airplane came to me this way, which is why my priority 1 is an EDM-960 or similar.

[wslade2]

Ditto. Mine came without engine monitor.

I’m in the process of installing engine monitor. Reason: had some cylinder work, mag work and Fuel system work done. On run up engine very shaky. We kept going over and over everything we did and couldn’t find a fault....until I leaned over and burned one hand while the other rested comfortably on neighboring cylinder. Problem was cold cylinder. Long story short. There was the smallest amount of grit plugging injector. Definitely work on your cold spark plug problem. I could have saved a couple weeks if I had installed engine monitor from getgo.

Also, 1 inch of movement seems like a lot. Look carefully to see if you engine is sagging. Motor mounts (rubber cushions) may be shot. I know cause just had that problem too.

Couple years back, had similar problem with my new to me 182 at take off. Wasn’t terrible but could feel it. It was everything together: bad mounts, 1 weak mag, high resistance plug, one cylinder had induction leak (caught those last three via engine monitor) and topped it off with dynamic balance. Wah-la! Problem solved.

[JAG]

Everyone has added great advice on digging further into #2 cylinder - so I won't dogpile on there.

Just a couple more thoughts on rough running/vibration. If your engine ignition, timing and mixture is all correct, you should not have any vibration. I can idle my rear engine at 675 RPM, and there is no vibration or beer spilling. Getting the engine set up right on the above items is important. A good A&P should be able to sort it quickly by starting with the simple things first; magneto timing, spark plug lead testing, spark plug testing (I would just replace with new ones...), cleaning and testing the injectors, checking for induction leaks, then performing the proper checks on the fuel system (SID97-3G).
Jeff