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Engine stumble

Simon Hitchen

Well Known Member
Hi guys, I’ve recently completed another annual inspection. FWF I checked everything, removed and cleaned the fuel injection nozzles, cleaned the spark plugs, checked the mag timing..all the usual things. Compression in all cylinders above 77.

I’ve flown 5 times in two days since the annual and on two occasions while turning crosswind at 500’ after take off the engine has stumbled momentarily. Normal operation followed for the duration of both flights. Certainly took the enjoyment out of them tho...

I downloaded the data file and for the more pronounced of the two incidents there is a 40 degree drop in number 1 EGT with an associated drop in Prop RPM (fixed pitch).

Any thoughts, advice appreciated, maybe unseen debris in the injector?

Photos attached, thanks guys.
6140177-C-D3-ED-4590-9-C64-0-F3-A82-C72-A50.png
 
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Sticking valve would be my guess. This happens when the valve stem, and or the guide, gets a build up of crud. Do a search for stuck valve.
 
That's a puzzle. My thoughts, so far:

Fuel: partially, momentarily clogged injector, as you suggested. Cleaning the injector is a low-cost thing to try.

Spark: What kind of ignition system do you have? Possible intermittent issue with one spark plug or ignition lead (or coil, if electronic ignition). Replacing both plugs and carefully inspecting leads and springs is a low-effort step to consider.

Air Flow: As has been suggested, intermittent valve sticking is a candidate. "Morning sickness" usually presents on a cold engine, however. Lycoming SB388C describes checking for carbon buildup (wobble test). Lycoming SI 1425 describes reaming. Mike Bullock posted an awesome description here: http://www.vansairforce.com/community/showthread.php?t=128673

I'll be interested in hearing what you find as you investigate.
 
75 RPM drop is significant, so it likely lost efficiency in that cylinder. I would expect a greater drop if it stopped firing. The fact that it only occurs when cold would point to a sticking exh valve as the other poster mentioned. The fact that it only happens cold make the sticking valve far more likely than spark or fuel. A partially sticking exh valve will often still let that cylinder produce some power and your data supports that. It is likely going up and down, but can't fully seat, creating low compression and hence the power and EGT drop off. Yes, a lean fuel shot will also do that, but I would expect to see it in all conditions, not just when cold.

Larry
 
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Wobble Test?

Engine type? Hours since new or overhaul? Have you done a valve wobble test? (SB 388, if memory serves)

I would investigate before further flight. I had a stuck exh valve inflight, the piston broke the head off the valve, and I flew home on 3 cylinders, with significant vibration. I was lucky that the piston did not get "holed", or the outcome would have been much worse.
 
Thanks

Thanks guys,

Upon examination I found quite a bit of blue dye on the number 1 injector leading me to believe maybe it was partially plugged. I cleaned it again and flew for thirty minutes. No issues.

The first time this happened it was on the second sector of the day, the engine was very warm.

Puzzling.

I?ll do the Sb regarding valve wobble just in case.

The engine has 1300 hours since new and has been flown very regularly since day1. It used to be Jon Thocker?s so very well looked after.
 
You mention cleaning injectors in your list of ?usual? things done at CI.
Some of us are of the school that if it isn?t broke, don?t fix it, including some of the injection gurus.
I have cleaned my injectors once at 800 hours. I wasn?t having any issues, but thought it was time, maybe overdue?
On inspection, there was absolutely nothing wrong with them other than screens where getting a bit of crud built up. The injectors themselves looked fine and cleaning and reassembly changed nothing in engine parameters (EGT).
If you find it was the injector, your problem could have been induced.
 
Thanks guys,

Upon examination I found quite a bit of blue dye on the number 1 injector leading me to believe maybe it was partially plugged. I cleaned it again and flew for thirty minutes. No issues.

The first time this happened it was on the second sector of the day, the engine was very warm.

Puzzling.

I’ll do the Sb regarding valve wobble just in case.

The engine has 1300 hours since new and has been flown very regularly since day1. It used to be Jon Thocker’s so very well looked after.

Your symptoms with a warm engine would make an injetor more likely. However, a blocked injector shouldn't show blue dye, as the blockage is usually in the smaller, first orrifice. You may have gotten some crud through the screen that was blocking the second, larger orrifice and that could cause staining. Be sure the screen wasn't lost when you cleaned them.

Larry
 
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Surgery

Today I removed both valve springs to check for stuck valves and found both the intake and exhaust valves in perfect working order. Smooth gliding motion for full travel. So that’s something. I’ll put it all back together and fly it in the morning.

I was thinking that the large bit of the fuel injector might have had some unseen crud intermittently blocking the orifice thereby forcing fuel out of the filter.

I’ll report further. At least I feel im not in immediate danger of having a stuck valve damage the engine. Fingers crossed.

And Jonjay, yes, at this point I’m pretty sure the issue was self induced... live and learn eh
 
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Although I agree with the poster who said "if it ain't broke, don't fix it", I think it would be very unlikely that you suddenly started getting a sticky valve precisely after you finished messing around with the fuel and spark part of the combustion cycle. That you found blue fuel dye staining where there wasn't any before would pretty much confirm where to look...injector.

Plus, you said you cleaned it again, flew and had no issues. You most likely reinstalled the injector wrong (bleed port oriented wrong, incorrect torque, etc.) the first time.

Why are you messing around with valves and encouraging Murphy to reassemble something incorrectly *there* and cause a REAL problem? Go fly around and see if it's gone.

And FWIW, if you're flying your airplane regularly, you're already putting a pretty good cleaner/solvent through the injectors...100LL. Leave 'em alone unless you have a reason to remove and clean one of them.
 
My lancair did this and it was a sticking valve. O-320 lyc.

I removed the valve spring and could feel the valve sticking in the guide. After reaming on the aircraft ( there is a lycoming procedure for this) it ran perfect.

Another symptom was a rough idle at times. It turned out my engine had been stored for years with preserving oil. I think that is what fouled the guides.

Good luck

Tom
 
Agreed

After a lot of reading and a lot of data analysis I?m in complete agreement.

I?ll follow the Lycoming procedure and ream the exhaust valves. I?ll report back.

Thanks
 
Although I agree with the poster who said "if it ain't broke, don't fix it", I think it would be very unlikely that you suddenly started getting a sticky valve precisely after you finished messing around with the fuel and spark part of the combustion cycle. That you found blue fuel dye staining where there wasn't any before would pretty much confirm where to look...injector.

Plus, you said you cleaned it again, flew and had no issues. You most likely reinstalled the injector wrong (bleed port oriented wrong, incorrect torque, etc.) the first time.

Why are you messing around with valves and encouraging Murphy to reassemble something incorrectly *there* and cause a REAL problem? Go fly around and see if it's gone.

And FWIW, if you're flying your airplane regularly, you're already putting a pretty good cleaner/solvent through the injectors...100LL. Leave 'em alone unless you have a reason to remove and clean one of them.

First, installing the injector without clocking the bleed will have no impact on running performance, only how/if fuel bleed out on shut down. Second, Lycoming requires a 500 hour check for tight valve guide clearances, so checking this is never a bad idea and FAR more likely to find problems than introduce them.

I do agree that removing injectors for preventative cleaning is not required. Clogging tyically occurs from crud breaking off somewhere else, not slow build up on the injector itself.

Larry
 
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Today I removed both valve springs to check for stuck valves and found both the intake and exhaust valves in perfect working order. Smooth gliding motion for full travel. So that’s something. I’ll put it all back together and fly it in the morning.

I was thinking that the large bit of the fuel injector might have had some unseen crud intermittently blocking the orifice thereby forcing fuel out of the filter.

I’ll report further. At least I feel im not in immediate danger of having a stuck valve damage the engine. Fingers crossed.

And Jonjay, yes, at this point I’m pretty sure the issue was self induced... live and learn eh

A smooth moving valve in the guide when cold is not a negative indication of a sticking valve issue. You need to perform the wobble test to determine the valve/valve guide clearance. THey typically don't stick when cold. The stick at specific temps in the mid range, but lower than normal operating temps. The valve heats a bit faster than the head/guide and this is when the initial sticking occurs.

Not saying this is your issue, only that your test doesn't rule out a sticking valve.


Larry
 
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Lycoming requires a 500 hour check for tight valve guide clearances

The Lycoming manual says

400-hour inspection...
Valve Inspection ? Remove rocker box covers and check for freedom of valve rockers when valves are closed. Look for evidence of abnormal wear or broken parts in the area of the valve tips, valve keeper, springs and spring seats. If any indications are found, the cylinder and all of its components should be removed (including the piston and connecting rod assembly) and inspected for further damage. Replace any parts that do not conform with limits shown in the latest revision of Special Service Publication No. SSP1776.

it didn't say "remove both valve springs", which is what the OP did. Maybe I don't understand something, but it seems Lycoming recommends "inspect", whereas he did some "disassembly/reassembly".

And, again, I find it highly unlikely that right after someone messes around with the fuel system, an engine stumble that didn't exist before is not related to that messing around. That seems at least initially confirmed by the stumble going away after he disassembled and reassembled the fuel injection components. I think, barring any further evidence, that he took an unnecessary risk by disassembling the rocker springs and such while taking the shotgun approach to problem-solving, that's all.

BTW, thanks for the info on the injector nozzles...I didn't know that, mine are different (Precision Airmotive) and from what I can see don't have any bleed port of any kind.
 
Clarity

Perhaps I should explain,

I value the opinion and experience of guys on this forum, that’s why I post here and have done for 10 years.

Of course it’s not my only source of guidance. I’ve consulted very competent local engineers and we’ve gone through all the symptoms and it seems some of the more subtle sticking valve cues were present before I performed the annual FWF work.

I don’t take invasive engine work lightly and exhausted all the non invasive troubleshooting steps first. BTW, The very brief occasional engine stumble occurred again after I’d gone over everything again.

Next week I’ll go through the Lycoming service instruction with a very experienced mechanic and see if we can cure this occasional stumble.

I appreciate all the replies and just wanted to add reassurance that I’m not going to unnecessarily mess with this engine nor do I take a “shotgun approach” to problem solving.

Any further ideas welcome!
 
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Perhaps I should explain,

I value the opinion and experience of guys on this forum, that?s why I post here and have done for 10 years.

Of course it?s not my only source of guidance. I?ve consulted very competent local engineers and we?ve gone through all the symptoms and it seems some of the more subtle sticking valve cues were present before I performed the annual FWF work.

I don?t take invasive engine work lightly and exhausted all the non invasive troubleshooting steps first. BTW, The very brief occasional engine stumble occurred again after I?d gone over everything again.

Next week I?ll go through the Lycoming service instruction with a very experienced mechanic and see if we can cure this occasional stumble.

I appreciate all the replies and just wanted to add reassurance that I?m not going unnecessarily mess with this engine nor do I take a ?shotgun approach? to problem solving.

Any further ideas welcome!

Good deal...it wasn't apparent from those earlier posts that the symptoms existed prior to your annual, nor after the second R&R of the injectors.

Good luck, and report back what you find!
 
Photos

It?s been an evolutionary discovery process for sure.

I?ll take some photos of the process and post the findings.

Thanks guys.
 
The No 1 EGT fell when you had the rpm drop.
I would have expected EGT to rise if the valve was sticking. The burning gas would have been passing the valve into the exhaust pipe for most of the cycle. That to me points to an injector as being a more likely problem.
A spark plug problem would also most likely lead to raised EGT.
 
The Lycoming manual says



it didn't say "remove both valve springs", which is what the OP did. Maybe I don't understand something, but it seems Lycoming recommends "inspect", whereas he did some "disassembly/reassembly".

And, again, I find it highly unlikely that right after someone messes around with the fuel system, an engine stumble that didn't exist before is not related to that messing around. That seems at least initially confirmed by the stumble going away after he disassembled and reassembled the fuel injection components. I think, barring any further evidence, that he took an unnecessary risk by disassembling the rocker springs and such while taking the shotgun approach to problem-solving, that's all.

BTW, thanks for the info on the injector nozzles...I didn't know that, mine are different (Precision Airmotive) and from what I can see don't have any bleed port of any kind.

It is in an SB somewhere. They recommend checking valve guide clearance via the ?wobble test? every 500 hours. It is not in the manual
 
It is in an SB somewhere. They recommend checking valve guide clearance via the ?wobble test? every 500 hours. It is not in the manual

I thought it was 1000 hours now with new, improved valve guides, and it's a Service *Instruction*, not a Service *Bulletin*. Is that info incorrect?
 
Update

Hi Guys,

Thought I’d update you with the results.

My exhaust valves were too tight in their guides with a little resistance felt as the valves approached their closed position. I reamed all four valve guides with the appropriate Macfarlane reamer which when lubricated allowed an almost frictionless, unhindered valve operation. I greased the reamer before inserting it into the valve guide and upon retraction wiped the grease onto a paper towel. Each time I found no metal just black carbon build up residue.

Everything was cleaned, all cylinders boroscoped, valve faces and seats visually checked for colour, condition and fit and everything re-assembled. I removed the hydraulic tappets, cleaned them and returned them dry to assess valves tappet clearance. All clearances were mid range of the allowable dimensions. Everything looked remarkably clean internally but I do run this engine at 50 degrees lean of peak in the cruise and aggressively lean the engine for all ground ops.

With everything buttoned up and following a comprehensive ground test I flew the aircraft for an hour. The engine performed beautifully. No more morning sickness, very smooth idle, less vibration at full power and no stumble. Even seems to have more power.

Comfortable that it was performing well my wife jumped in (that’s trust for you) and we flew to a few social events, 5 flights and 2.5 hours total flight time finishing at 23:30.

Very happy with the results. I learned a lot about the top end of the engine. Well worth doing Lycoming service instruction 1425 A if appropriate. I did need a few specialised tools but the local maintenance facility were happy to loan them to me. Large Pizza accompanied their return. Everyone’s happy.

Big thanks to Mike Bullock for documenting the procedure so clearly on a previous post, sure helped with the studying in the days before heading to the shop.
 
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