Engine Problem Troubleshooting

[NoseDragger]

Hello all,
I am a new/old member, I previously owned a 49 A model. A few years ago I went to A&P school to add those certificates simply because I like working on my own airplanes. I was 57 when I graduated in 2018, so needless to say, I was the oldest in the class! I can say, I am more proud of those certificates than my ATP certificate! Definitely more work.

As a graduation present to myself I bought a 1966 H Model 172 as a project and went through it and mechanically restored it (it was my Covid project!). I went through the entire airplane and replaced/overhauled just about everything with the intention of having something to fly that is safe, dependable, and economical to operate... in that order.

The engine was overhauled with new TCM cylinders and has about 400 hours on it. A few months ago I had my first stuck valve on #2. I bought a reamer (the correct size from the MM) and used the rope trick, dropped the valve and reamed the valve guide. This seemed to have done the trick at first. I have flown it a little over twenty-five hours since, but over the last five hours or so I have been getting an occasional engine stumble when full power on climb out. It is almost like it is getting a gulp of water while in a nose high attitude during climb, but it is hangared and I have never even gotten a drop of water in my fuel samples. Until today, the stumble/roughness has only lasted about a second or two, but was enough to get my attention. Today was the last day before I start the annual and I tried to get one more flight before getting to work. This time on climb out the stumble came back and lasted for a few seconds. Not severe roughness, but not smooth either. I did a quick mag check with no unusual results. Since I had just departed my airport I turned around and entered downwind for a full stop.

The airplane has otherwise been running smoothly, static RPM has been normal, and I always lean in taxi (lean until the fuel flow indicator shows a small drop), and I lean in cruise (lean until rough, then enrichen just enough to smooth out and bring rpm back up, also while watching egt).

So right now, my suspicions are a mag problem that is intermittent (lower probability), or a sticky valve (higher probability). I guess there is the possibility of a bad/intermittent spark plug, but that doesn't seem very likely. I will be cleaning and checking them for the annual. They are around 400 hours, so maybe it is time to replace?
I had a similar problem on a 172 with the H2AD engine and it turned out to be a bad magneto coil.
So, I know there is a lot of knowledge in this forum, any guess on what I should troubleshoot/inspect?
I hate to just start reaming valve guides hoping that will solve the problem.

Thanks,
David

[johneeb]

David,
Today, at least in the Chicago mid-west region, was an excellent day for making carb ice. While my 170 does not have a Carb the family 180 does and it just loves to make ice and act the way you describe"
Johneb

[NoseDragger]

That is how it was behaving, but it has always been at full power on climb out. I have always associated that with low power. And, yes, today in the PNW it is overcast, cool, and probably a bit humid.

[Bruce Fenstermacher]

David, welcome back. I was 65 when I got my A&P and 68 when I got my IA in 2025. I worked harder and longer to get them than my ATP.

The thing about sticking valves is having one means all the others are just likely about ready to stick. I've had quite a few stuck valves over the years and it is my practice to check all the others valves if I have one that sticks. Can't say your symptoms are typical of a sticky valve more than it was carb ice. A stuck valve tends to show itself when the engine is cold, at low throttle rather than warmer at take off with full power. You can get carb ice on departure.

[GAHorn]

Anytime a stuck valve is removed to ream a guide, …is a great opportunity to also test the valve springs. Weak / under-strength valve springs are more common than one might think. (I have a bag full of springs that failed to meet spec over the years. An intermittent “stumble” was the symptom which led us to finding them.

IMG_0073.png

[NoseDragger]

Thank you, all good info!
My stuck valve was found on first start one morning. It was actually the last oil change, I was warming the engine up. Interestingly, the prior flight I distinctly remember thinking about how well it was running! I reamed that #2 cylinder and also #4 since it has had lower compressions.

The weak springs are definitely something to think about. I had to do a quick research on how to measure that. I found a couple of cheap spring measuring tools that can be used in a vice. I will have to think about that. I may wait until after annual and see if it repeats. Since I was at high power, I did not apply carb heat to see if that would clear it up.
Now that I am thinking about it, the other instances were on similar cool overcast days, and it happened around 2000’ like today. So that could very well be carb ice related.

[GAHorn]

Carb ice is something I’ve experienced in Continentals, although mostly in O-200 engines. The Cessna 150’s I flew early in my career frequently developed carb ice. They made excellent primary trainers in so many ways, and that was one of them…. they were “almost”… “Carb-Ice-Demonstrators” in the Houston/Coastal area where I did most of my early training/teaching.*

The O-300 engines were in the 172’s we had available at the places I frequented…. as this was in the 1970-74 period and the Lycomings had only recently been introduced to thse models. Neither of them were the ice-factories the O-200s were…. In Fact, I personally cannot recall having a carb—ice event in a Lycoming…. Ever.
If I had carb-ice in an O-300 it was so-rare as to not be memorable….but, regardless, I regularly Check-For-Ice before each/every throttle-reduction below cruise-speed on that engine. It is suspect in every mysterious engine failure/un-planned landing.

*(anyone who’s heard or read my recollection about my pipeline-friend Ricky Burdeaux may find carb-ice to be a very real hazard. I cannot think of the subject without that humorous corollary.)

viewtopic.php?p=8574&hilit=ricky+pipeline#p8574

[NoseDragger]

Thanks for all of the good info!

I typed up a response a few days ago, then it disappeared before I could post it> I seem to remember that happening a few times when I belonged to the group in years past.

Bruce, the stuck valve happened on startup when I was trying to warm it up for the last oil change. Funny, the prior flight I remarked to myself about how smooth the engine was running. I reamed the exhaust valve on the #2 affected cylinder (It was stuck hard open), and the adjacent #4 cylinder. #4 has been showing lower compressions, so I thought it may not hurt to do it as well (not that the two are necessarily related).
Last year I lapped the #3 valve in place for not sealing well, worked great.

I generally run at 70% and lean until the rpm just drops, then enrichen enough to bring it back up to smooth it out (maybe 3/4 of turn on the mixture). I also look for the egt to drop from lean peak to that enrichened state and note the fuel flow while I am doing this. I get the same results by ear as I do from the gages. To make a short story long, I am running the engine just slightly rich of peak, as I believe most recommend for this engine.

I checked the fuel vent line to make sure it was clear to rule out pulling a vacuum in the fuel tank. I have one vented cap as required... not sure of the best way to check that it is venting properly. I may order a second vented cap as a "just in case" measure. I don't believe that this would be wrong to set it up this way.

With all of this said, I have mostly convinced myself that it was carb ice forming. Each time it has happened was on cool overcast day, and I was climbing through roughly 2000'. I am wondering, if possibly my Brackett air filter became dirty and is pulling a bit more vacuum that would create icing conditions in the induction system? We do have some surprisingly dusty conditions here. Probably reaching, but something to consider.

In the medical community they say "When hearing hoofbeats, don't look for zebras", or something to that effect. I feel like I am looking for zebras right now.

George, that was an entertaining story! I am curious about how the weak valve springs make an intermittent stumble, it would seem that it would always affect the engine at some higher RPM. Maybe just one of those mechanical mysteries?

Anyhow, thanks all for the info.

[Bruce Fenstermacher]

Thanks for all of the good info!

I typed up a response a few days ago, then it disappeared before I could post it> I seem to remember that happening a few times when I belonged to the group in years past.

if you type up a reply and you don't hit submit, it will disappear. If it takes you more that 30 minutes to type the post before you hit submit, it will tell you in read you've exceeded the time allowed and you will have to submit the reply again. Most times the second time you hit submit, your post will be saved to the forum. IF you wait about an hour, your session on the forum will be terminated and you will have to log back in and your post will disappear and not be submitted.

[GAHorn]

..
George, that was an entertaining story! I am curious about how the weak valve springs make an intermittent stumble, it would seem that it would always affect the engine at some higher RPM. Maybe just one of those mechanical mysteries?

Anyhow, thanks all for the info.

I don’t have a “scientific explanation” of how weak valve springs manifest in a “stumble” …other than to suggest a reminder that engine-functions occurring at high-speed ..(if you can call 2450 RPM “high speed”) …. require a smoothness-of-function of All the mechanisms within a fairly short time-frame.
A “sticky” valve… one that has already been in-service for a half-million cycles… and perhaps coupled with springs that have sat idle…in compressed…or partially-compressed physical-state ….perhaps for weeks at-a-time…. may not properly function (as poppet-valves were designed) exactly as intended.

I Do know that when the springs were replaced, My engine ran smoothly again…so I presume that was the main problem.

Were weak springs the ONLY contribution to the intermittent operation..? Perhaps. Perhaps not…because the valves themselves were also “renewed” at the same time. I suppose it’s possible that the weak springs might have been re-used and those (now clean/renewed) valves might have been sufficient to cure the problem…even if re-used with the weak springs…. It’s highly conjectural….but the entire valve-train system must function correctly and within the short time-frame allowed in order for the engine to perform.

What can I say..? … Other than to hope to help you …while I’m not actually “present” to experience Your Engine…and then hope to be sufficiently “lucky” in finding the culprit.? I can only offer my experience with what worked in a real Example of my rough-running engine.

You had a sticky-valve with a stem-to-guide clearance iossue. Couple that also with a week spring-set….and what have you? Would stronger springs have prevented the stuck (or slow) valve movement? Does a “slow valve closure” that is intermittent cause an engine to “stumble”…?
Does your engine havve Valve “Rotators”…? Is that perhaps a cause for a “sticky valve” only when the valve is rotated to a particular-relationship to the guide?

[NoseDragger]

George, just to be clear, I was not questioning the validity of possibly having weak springs, only being curious about the actual mechanism of disfunction.

I have already researched how to measure spring compression and have found a couple of measurement tools that don’t seem to be too pricey. The next time I have to drop a valve, I plan on purchasing one in order to check them.

I also looked at some older threads concerning dual vented gas caps and ordered a second one last night. I feel like that may be a bit of a stretch, but certainly a possibility, and it gives a little bit of redundancy as far as venting of the gas tanks.

[GAHorn]

A “cheap and crude” method to test compression springs… is to compress the spring to the specified test-length on a weight-scale (digial bathroom for example…. and see how much is registered on the scale.


I.E. : cut a piece of pipe the specified test-length of the spring….(for example 1.168”)… and stand it on the bathroom scale. Now take the spring to be tested (say, PN 625958 …. and “bear-down” upon the spring compressing it…. until the spring is the same length as the pipe…. and read the scale.
In such an example, a flat piece of steel laid across both ..pipe and spring…. observing the scale read-out…as the spring is compressed the read-out will increase more and more …until the scale suddenly “jumps” to a much-higher value as the pipe begins to take-on some of the pressure. That reading seen immediately prior to the “jump” is the value of interest.

Another method is to use a “fisherman’s scale” to hook thru the spring, pulling the scale which compresses the spring to the specified length…the scale will provide the number to be observed.

I once checked an oil press relief spring by the second method using a bench-vise to hold the spring compressed to the specified length….. then observed the amount of addt’l effort needed to compress the spring further.
Not an “approved” method… but one which satisfied my curiosity.

(I’ve also found a postage-scale useful to test the amount of effort needed to activate a stall-warning Lift-Detector…. (specified at 5 grams IIRC). In that way I was able to discover the previous owner of my 170 had substituted a physically-Identical….but incorrect…..computer-micro-switch for the “correct” switch. (The stall warning horn activated properly on pre-flight ….and Annual Inspections….but it NEVER activated in Actual-Flight during a Full Stall. The computer “floppy-drive door-microswitch activated at something like a HALF-POUND! Very sneaky of the previous owner, tho’, …heh ? )