Comanche 180 - wandering RPMs & roughness

[Quint Van Deman]

Fellow Comanche owners--

I've been flying my 180 for about 4 months now and love it. However, it does exhibit one characteristic that troubles me a bit.
For starters the engine is about mid time (1200), had a recent top overhaul (50 hrs ago), but is ageing in calendar years (19).

She runs great through start, run up, take off, and a 25/25 climb. However, once I get to the top of the climb (usually 6' or 7'), I reduce power to 75% per the tables (maybe 22.5/2400/requiring 3/4 throttle) and do my best to lean by the classic "lean till it runs rough, then enrichen till smooth, and then a little more". At that point I have no issues with power (getting about 135 knots true out of a 180), but I notice 3 things:
1. As an engine guy something just doesn't sound or feel right - there's a slight vibe and the noise of the engine just seems off.
2. The tach continuously wanders up & down +/- 50 rpm, not enough to be alarming, but enough to be concerning.
3. Adjusting the mixture and/or carb heat doesn't really seem to have any effect.

As I reduce power for the decent the symptoms persist or get worse to the point where I was actually quite concerned one time when on a 20"/2400 downwind @ 1000' full rich, the roughness got a worse and for one quarter of a second the engine just seemed to stop firing, but resumed before I could even blink.

The last part of the equation is that after graduating out of the token cessna 150 for my initial training, I've flown nothing but fuel injected birds with a fuel flow gauge (obviously the 180 lacks both). So I will say that my leaning & carb ice skills could be a little "rusty". Does anyone have any thoughts? The engine was given an extensive pre buy 4 months ago by a trusted mechanic:
1. Has my questionably accurate leaning lead to fowled plugs?
2. Could I be getting carb ice that often (in clear air at 70 deg OAT)?
3. Could I have an ignition problem (despite normal run up and mag check)?

Any thoughts would be greatly appreciated as this little nuance has kept my from entirely trusting the old 180 a bit recently.

Thanks.

[Hansmeister]

You and your trusted mechanic might think about:

1. Dynamic prop balance,
2. Prop governor overhaul,
3. Switch to fine wire plugs (yes, I know that they are expensive).

[N3322G]

I would add mag timing to Hans list and might put fine wires first
... and welcome to Comanche ownership.

[Quint Van Deman]

Thanks guys. After I posted last night I was reading some of the other threads and realized I'd left off one other piece of info.
When I push the throttle to the firewall for takeoff the RPMs shoot well past the marked red line (maybe by almost 200 rpms).
It hasn't worried me because just about every airplane I've flown has done the same thing.

As for the noted suggestions, they all make sense in terms of the rpm wandering, but any thoughts as to the engine roughness?

Thanks much for the feedback.

[md11flyer]

Hi Quint. I am no expert, so here are my two cents. Check when was your prop governor overhauled? 200 rpm overshoot is not normal... in my books anyway. I would also check when the prop was off last...might be in need of an overhaul. vibration might be coming from the prop.(blade /clamp loose?) overshoot might be from the prop stickiness/ corrosion...

Again sounds like you need to look at the prop for clues... the slight surge in the engine could be from a bit of water finding its way
from a wrinkle in the fuel bladder.

Good luck

Gary

[Zach Grant L1011jock]

Quint,
Make sure you get the correct info. Start with the tach. Is it showing the correct rpm and correct swings. Can you verify the overshoot with engine noise? Don't chase a problem you don't have first and formost. As for if the indications are true, the above comments are accurate with the addition of looking at the oil line that runs from the governor to the front of the engine that supplies oil to the prop. Sometimes these become sludged up, and need a cleaning. Another issue may be valve issues. I know you said it just had a top, but were the valves lapped correctly, and or how was it run before you got the engine. Do a wobble test as per Lyc SL388 and check for freeplay. That might be more of an issue than anything else for vibration especially when reducing throttle but not RPM and having an incresed vibration. One last thing. If you have an original prop on the airplane, you might be money ahead at this point to just replace it with a new Hartzell or Mac prop.

Zach

[Quint Van Deman]

Thanks Zach & all--

As for if I can hear an overspeed on takeoff, no, everything sounds like I expect it to, but I probably don't have enough hours in the Comanche (40ish) to have my ear finely tuned to her noises.
I think you're right though in that I'll try to get a digital tach to start with, to isolate the indication from the occurance & go from there. The oil line tip is also a good one, having read several other posts, should I see a dip in oil pressure as the tach approaches 2700? Is it noticable if your eyes (or better yet the right seat eyes) are on it at the right time? To answer one of the other follow up questions, it does not have the original prop, it's a 3 blade McCauley, but it's been on there for a while (700 ish hours but 10+ years). I'm going to take her up today armed with some of this new information to see if I can further diagnose things. Thanks again for all of the advice.

-Quint

[Zach Grant L1011jock]

Quint,
If you are overspeeding by 200 rpm before the gov grabs you will definitly hear it, and probably feel it! Getting a good reading from an optical tach would be a great place to start. Honestly, you would be very hard pressed to see the oil press change at takeoff. It is not as abrupt as when cycling the prop, and is a gradual increase in oil flow to the prop, so the draw down is not as fast allowing the oil press to recover, and thus not be seen on the gauge. As for the Mac prop, if you have a loose blade or a track problem that could be your vibration. Also, it is very sensitive to how well the leading edge is dressed, with even a rough paint transition causing a change in vibration. I would also be very suspect of your engine mount isolators. If they haven't been changed in the last 3 years or 500 hrs, whichever comes first, I would change them.

Zach

[N3322G]

Quint,

There are handheld prop strobes that can check the actual RPM on a prop - perhaps you could borrow one from a local prop shop.

We have one and when we use it aloft, we always have two on board so that one can look for traffic. Of course, it also works on the ground but being in front of a plane or even close at the side at 2700 rpm makes me nervous, so we check while aloft. Too bad you aren't closer than the east coast or we could help with this tool.

[Quint Van Deman]

So I took the 180 up for a spin yesterday armed with a better idea of what I wanted to check out, and here's what I found:
1. The prop didn't overspeed. During both a ground run up and a takeoff run, the RPMs gradually increased to 2700, stopped, and then actually backed down in the neighborhood of 2675 or so. I was probably just imagining things with respect to an overspeed, but I'll certainly be very attentive to the numbers from this point out.
2. My original problem seems to still exist in that the tach neddle wobbles +/- 25 rpms and I get some occasional engine roughness. When if roughens I usually give it a shot of carb heat, and adjust the mixture and everything seems to be fine....but troubling none the less.

I will see if I can locate an optical tach to isolate the rpm wandering, and might have to consider putting in a JP engine monitor to help better lean the engine. Right now I have little confidence in the EGT gauge that's in there, and the "old school" method doesn't seem to be working well for me.

Thanks.

[Zach Grant L1011jock]

Old school should work regardless. You may have a carb issue. Just one more thing to look at.

[skipsouthernsky]

Dear Quint,

My two cents would be to open up the engine logs and look to see when the last time the mags were overhauled. If not within about 3 or 4 years, I would take them both off and have them opened up and checked thoroughly. You'd be surprised what you find inside some mags (carbon tracking on distributor caps, worn bearings with side play, pitted points, etc.).

Some might suggest doing a mag check in the air, but I would advise against that since misfiring mags tend to allow unburnt fuel into the exhaust and then explode when returned to the good mag.This can damage the exhaust system.

The tach wandering you are experiencing is most probably due to the tach being old and not in pristine condition. Most mechanical tachs are considered good if they are within 1% and sometimes 2% accuracy. At 2500 RPM that would be +/-- 25-50 RPM accuracy. The wandering could also be from a worn or dirty tach cable. I would get ahold of a digital hand-held tach indicator and check what the engine speed is really doing and not worry so much about the aircraft tach at this time.

I would be more concerned about the engine running intermittantly rough. The reason for that needs to be identified. Problems like that will usually go away with a good thorough 100 hour inspection. This would pull in all the items needing attention such as harnesses, spark plugs, fuel filters, mags, air filter/induction system, compressions, etc. There could be multiple problems which should become more clear with such an inspection. My understanding is that you haven't owned this plane for too long and probably haven't been through the inspection process in detail. Find a good shop and get this done. It will be good for your peace of mind and your understanding of where various components of your engine stand at this time.

Doing a shotgun approach and thinking maybe this and maybe that is not a very efficient way of finding your problem. Especially if there are multiple problems. I seriously doubt if your problem has anything to do with fuel flow or carb ice.

Sincerely,
Skip Dykema

[Quint Van Deman]

Thanks Skip.

To keep adding to the mystery, I flew today, and while the tach continues to wander a bit, I didn't have any signs of roughness at all (which I'm not discounting as a sign of resolution, but it was still nice). The only thing that I did noticeably different was that I drained the sump for about twice as long on each side. Last time I encountered the roughness it only occurred after I switched tanks (not for about 10 mins after), which could have been coincidental, but at the time I was very much grasping at straws. I will admit that I do not regularly catch what's drained out of the sump, but it's something that I'll likely start doing as well.

To your specific suggestion, are there items that are covered in a 100hr that aren't covered during an annual? I turned my prepurchase inspection into an annual when I bought the 180 back in Feb, so assuming that was an inclusive inspection of the items you mentioned, it's at least been done within the last 4 months/40 hours.

[N3322G]

Quint,

You added a significant detail today wrt the draining. I don't know if it is different for singles but the Twin recommended draining procedures include draining before the first flight and after the last of the day. We catch the first drain and make sure it is clear. We do not catch the post flight drain - usually.

Also, we had the old tanks vacuumed at one annual and got a lot of crude out. There are new bladders in now with no wrinkles to catch debris.

If a plane sat for a long time - sometimes it just needs flown to get rid of gremlins.

I'll be interested if the problems are solved with more draining and flying. Please keep us posted - we all learn from each other.

[skipsouthernsky]

Dear Quint,

I hear you about not catching the fuel that you are draining during preflight. I think all the single Comanches are like that and it is impossible to be inside the plane pulling the fuel release handle and outside under the belly to cach what is coming out when by yourself. What I did many years ago was to take a fuel sample cup (one with the metal rod that sticks up out of the center of the cup) and with a pair of pliers put a 90 degree bend at the end of the rod about a quarter of an inch long. I then figured out about where it needed to hang so that the fuel would drain out of the belly tube and into the suspended cup. Then take a drill and drill a 1/8 inch hole at the location and insert the cup's rod into the hole. The cup should hang just at the end of the tube.

It's still a pain to crawl under the belly and then go into the cockpit and drain the fuel and then back to the belly to observe the fuel in the cup, but worth it so you know what is going on with your fuel system.

I have to wonder who did your most recent pre-purchase/annual inspection. Some of these are better than others. Did the shop mostly work for you or the seller? Interesting. An annual inspection includes a 100 hour inspection along with checking the currency of all the AD notes. Although you can have a 100 hour inpection done any time you want, most people wait for the annual and have it all done simultaneously. It takes a mechanic with an inspection authorization (IA) to sign off an annual inspection, but an A&P (mechanic with Airframe and Powerplant certificate) can sign off a 100 hour inspection. For part 91, 100 hour inspections are not required in between annual inspections. For example, if you fly say 300 hours a year, you would only be required to have an annual inspection done once a year along with keeping current with any AD's that come due during the interum. On the other hand, you might want to have 100 hour inspections done every 100 hours during the year and that can be done by an A&P alone. An IA would only have to sign off an annual once a year. From my experience, if you fly multiple 100's of hours each year, then 100 hour inspections are very desirable and of course not needed if you fly less than 100 hours per year.

If you are curios about what a 100 hour inspection entails, then go to the home page of the ComancheFlyer web-site and look for the online Comanche Maintenance Manual there for the benefit of ICS members. You will find a 100 hour checklist in the manual maintenance practices for the Comanche. Probably more detailed in depth information than you ever wanted to know. Look at the items listed under engine and propeller and think to yourself if you think all these items have been checked on your plane recently. Food for thought.

Sincerely,
Skip Dykema