Al O’Donnell:
I have a 1980 Sundowner with an engine that has 500hrs on it. I have flown it now about 125 hrs. Twice while I was cruising at altitude, the engine momentarily seemed to labor, and ran rough for probably no longer than about 5 seconds… seemed more like 5 minutes.
Each time I was cruising at about 5500 feet on a clear day. One was in the Summer and the other this month. I was about to pull the carb heat when it cleared. Each time I had just filled up the plane about 30 minutes prior to the incident. I had checked for water in the tanks and drain… no indication before take off. Is it possible some water was there, but did not migrate to the low spots before take off? Is there any other reason this may occur? Other than those two times, I have had no problems and all temps etc appeared normal. I usually run a bit rich on cruise, about 100-125 deg at 5500 or above rich of peak.
Ed Fitchett:
It sounds like the “carbon build up on the valve stem” issue, covered under mandatory service bulletin 338 (SB338), but I’m not sure it applies to whatever engine you have. Although you have an engine with only 500 hours on it, the SB suggests it be done every 400 hours. Go to www.prime-mover.org/Aviation/Bulletins/sb388.html – mandatory only to
commercial operators – and check the threads on BAC and MM.
I had the same thing on my 80 Sierra’s IO-360; a momentary hesitation every once and a while. I described it as a burp, just as you said – like it had just digested a bit of water out of the fuel system – but each time it gave me the willies. My A&P suggested I either do the “wobble test”, which tests for valve stem clearance, or forgo the test and just put the test money towards cleaning the valve stems / guides prior to what he suggested MIGHT become a full valve seizure. I forget the details, but if a certain cylinder valve sticks in a certain way, it means engine failure.
I did have one stem badly out of tolerance; and after the fix, it has not happened since. The previous owner spent all sorts of time and money looking at fuel and ignition problems prior to me buying the plane, but never found the cause. I suggest you talk with your mechanic about SB338.
Bob Steward, A&P-IA:
Classic symptoms of a sticking valve. Time to do the Lycoming recommended SB 388B valve wobble check (every 400 hours), and see if you have a problem developing.
Jeff Bryant:
I was planning on buying the tool for SB388 from Spruce. I talked with the folks at Ly-Con about doing the valve check at 400. They told me that because I had new cylinders installed at overhaul last year, that a new material was used in the guides; and that the SB 388 would not be necessary.
I do not believe this is true. I think maybe they are confused. Have you heard anything like this before? As I read the S/B, it is required for all engines at 400 hrs, regardless of when the cylinders were purchased. I am going to buy the tool anyway, but thought maybe you had heard something.
Bob Steward, A&P-IA:
Lycoming changed the spec on the exhaust valve guide (again) a few years back, and they now call them “high chrome”. A very knowledgeable fellow that had the old ones and the new ones tested said that in going from NO CHROME to some ridiculously tiny amount (.00x%) that they definitely WERE “higher” than zero.
You can tell if you have them on new assembled cylinders from Lycoming, by a small “C” stamped on the valve cover flange at the “top” of each cylinder. Bare guides can be inspected for a slight chamfer on the top edge to indicate they are the later style.
Superior and ECI adopted a wait and see attitude on the guides, and have more recently changed their formulation as well.
Lycoming says that the interval for SB 388B can be EXTENDED to 1000 hours on those engines with “high chrome” guides. My experience is that they have the same problems as the “no chrome” guides, and that the carbonization and the wear are related to CHT more than alloy. Keep the CHT below 400 F all the time, or pay for it with accelerated wear.
So YES, please check your guides, regardless of which ones you might have. You can buy the tool from Spruce
http://www.aircraftspruce.com/catalog/topages/valvewobble.php
Or rent it for about 1/4 the purchase price from Approach Aviation in MA.
http://www.approachaviation.com/
Editor’s note:
It is feasible that you are getting a small amount of water, but it is quite improbable. The 19/23/24 are relatively very good about migrating water to the drain relatively quickly, though “quickly” could well mean ten or fifteen minutes. Part of the reason it is improbable is that you would have to get a great deal of water, for it to reach the level of the fuel finger screen (the fuel pickup point); probably upwards of a quart or more. You would certainly see so much water in sumped fuel, probably for a while. If the events occurred after fueling at two different locations/FBOs, I would say that water is virtually out of the question.
That means that Ed and Bob are probably right, and you are experiencing a sticking valve. While this usually shows up first as “morning sickness”, a roughness after start-up that slowly smoothes out, that’s not always the case. A sticking valve is a very risky event in our aircraft engines, due to the design and the masses/energies involved. I would advise against trying to just live with this. A valve head, stem, pushrod, or cam failure will result in a trashed engine, and probably worse if it leads to a forced landing.
I would also like to add that, in addition to running too-high CHT temperatures, running richer than necessary for conditions will contribute to carbon and lead build-up on the exhaust valve stems. If you want best economy, run as LOP as you can, from a roughness standpoint. This will cost you power and speed, but will help keep the engine internally clean. Any engine usually leans out the best when at full throttle, where mixture distribution is usually at its best. If you want the best balance of power, economy, and reduced deposits, run at peak EGT. You cannot reach an EGT that is literally too high, on an unsupercharged/non-turbocharged engine. Avoid running in the range of 40-50 degrees ROP; it has the fastest internal cylinder pressure rise, at a time when the rod does not have the best angle on the crank (to relieve the pressure). This is the mixture that is the hardest on your engine. If you want best power with a more gradual internal pressure rise, run 100-125 degrees ROP. For more on this subject, search BAC for leaning, EGT, and CHT.