Cycling PA-30 engine roughness

[Kevin Weidner]

Hello all,

We have a PA-30 that has an engine roughness that constantly cycles. It has about a 5 to 10 second interval and it will occur the whole time while flying. It will be almost perfectly smooth, then cycle through a slight, but noticeable roughness, then smooth again. It gets smoother once the engines are leaned out (100 rich of peak, 2400 RPM, 22", any altitude), but the 'cycling' time does not change.

I have noticed this problem for probably the last year or two. The airplane does not fly nearly as often as it should, but everything else has seemingly been working very normal. Both engines are in the 1600 to 1700 TSMOH, and less than 1000 STOH with new cylinders.

We can't tell which engine this is coming from, but it's just noticeable enough that it needs to be solved. I wouldn't think that it is particularly good reason to ground the airplane, but it does cause some concern in my mind.

Any ideas would be greatly appreciated,

Zac Weidner

[Zach Grant L1011jock]

Have you tried a different RPM? It sounds like a resonance thing to me. First thing I would look at are the lord mounts. If they havent been changed in a few years, they are probably the culprit. You really cant see the condition of these without removing them and inspecting unless they are totally rotten. If they are not in good condition, change them and then do a good dynamic balance on the props. I think this will solve most of the issues.
-Zach

[Kevin Weidner]

It does persist at 2200 and 2300 RPM, but we haven't flown often enough to experiment with different settings. I think replacing the mounts is a good place to start. The props have been balanced within the last 2 or 3 years.

[PJACKO]

HI Zac: just wondered if you solved the roughness problem and what it was. I have the same problem.

Jack

[Kevin Weidner]

We put all new Barry mounts on and it made a big difference in overall smoothness. However, we still have a very slight roughness that is consistent, yet intermittent, but it is barely noticeable now. The other day, it became very rough for a few minutes and smoothed out again after a while and ran great for the rest of the flight. It may have been some water in the fuel, but we could not tell what it was. We also cannot tell which engine is causing the roughness, but it seems to be less noticeable at higher cruise RPM's. 2400 is very smooth and 2300 is barely noticeable, but it get a little disconcerting at 21 or 2200.

Sorry I can't give you a good answer as to what is causing it.

[md11flyer]

Do you have an engine monitor? If you do you can download the stats and if its a water, fuel, or ignition problem it will show up. When is the last time you had a wobble test done on the exhaust valves? You could perhaps have a sticky valve.
This would also show up as roughness in the engine at start up that clears up after a few seconds.
If you don't have an engine monitor, I would advise you to ask for one for your next birthday or Christmas present. It pays for itself very quickly with regard to trouble shooting.
To check for ignition weakness, run the engine at 60% power in cruise flight, then lean it out till it starts to run rough, enrichen till smooth, then do a mag check run the engine for 1 min on one mag then the other. If your ignition system is not up to par it will show during this check. Sometimes its just an intermittent firing plug which will show on the engine monitor data.
Also if it is a defective exhaust valve it will show on the data as well.
My 2cents and distant guess is a possible exhaust valve, reasoning being your info on it being a very rhythmic roughness.
Of course just a guess but if you haven't done the valve SB within the last 500 thats a good reason to do it and eliminate
the valves as a possibility.

Good luck,
Gary

[N3322G]

Kevin,

Could be lots of different things and as you pointed out, it doesn't fly as often as it should.

When our twin sat a lot as we took care of family issues, we'd get in and have maybe 5 or 6 squawks - discouraging. Then we'd get back to flying weekly and miraculously, the problems disappeared. Not saying this is your situation but did have this experience.

We did take care to sump before flying until fuel was clear and then again after flying.

Might check the alternate air doors too. Rollers should be 1/8 inch off doors per maintenance manual.

We did have a circumstance that mimics what you describe. It was a piece of debris that intermittently blocked an injector. It was about the size of a piece of pepper - took two different mechanics before it was found -one out of town and then the home field guy. It was contamination from work on the fuel pumps. This is what I really like the engine monitor for now - - didn't have this at the time in the good old days.

I have to say, the intermittent left engine loss made the yaw a final approach quite memorable for me. Probably even more memorable to the folks standing on the side of the runway waiting to judge the spot landing. They scattered. Co-pilot ATP CFII Mom and owner said, 'center of the runway' and my reply was, 'I'm trying'. I did get it down on the centerline and under control. The story goes on but this is enough to keep you prepared in case it happens to you.

[Kevin Weidner]

I drained it this morning before I pushed it out of the hangar and found that there was about 2 ounces of water on the ground. I do not know which side, but it was in one of the aux. tanks, which is the tank we were on when it got rough, so that was definitely the cause of the unusual roughness for a few minutes. We will be taking a close look at the valves during the next annual. That was kind of my guess. It feels like some kind of valve resonance. Perhaps even a broken spring or something like that. At least I know what the unusual roughness came from.

I am not sure which SB you are referring to about the wobble test. Do you have the number for the SB, or maybe even a link to it? I think Lycoming has all the service information available on their site.

I am sure the infrequent flying has a lot to do with it, but it never sits more than a month without flying. I know 1 or 2 weeks is really all we should let it sit, but it's better than sitting for months.

Thank you all for the help and advice, and I will let you all know what it turns out to be, if we can determine any culprit.

EDIT: I think I found the SB on the Lycoming site. Is this the one you were referring to? http://www.lycoming.com/support/publica ... SB388C.pdf

[N3322G]

Kevin,

Glad to hear you found part of the problem. IIRC Lycoming looks for something like 5-10 hours a month to keep the engines happy.

If the tanks are kept full, we get little to zero water sumped in a year. That assumes, I've kept the fuel caps tight, the fuel doors gaskets in good shape and there are no leaks around the Aux gaskets that are partially under the engine nacelles - all of these have gifted us with water in the past.

At annual you might consider draining the tanks and sponging out any water. Vacuum if you have the right set-up, if not, sponging works just as well.

[md11flyer]

Kevin, yep thats the bulletin.

Gary

[Kevin Weidner]

Is it possible that the water is sitting in a low spot in the tank and just occasionally some of it is working its way into the lines? We had a leaking left aux tank cover and have since installed a new gasket, but maybe it's possible that some water has remained in the tank somewhere.

We were not aware of the valve wobble SB, but I will certainly be doing it soon. My thought would be that the engine that burns more oil (left in this case) would be the more likely culprit, since both engines have roughly the same compressions and the oil stays clean for about the same amount of time. This would lead me to think that the oil is being burned because of valve guide wear.

[Kevin Weidner]

We flew a couple of hours yesterday and we were unable to even notice any of the vibration. Maybe it just needed to do some flying, because it seems to have healed itself. Unless it shows up again, I don't think it's even worth worrying about.