Engine stumble

Simon Hitchen · #1 09-30-2019, 05:12 AM

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.

Tom Martin · #2 09-30-2019, 05:20 AM

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.

Tim Lewis · #3 09-30-2019, 07:57 AM

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/communit...d.php?t=128673

I'll be interested in hearing what you find as you investigate.

lr172 · #4 09-30-2019, 10:01 AM

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

PCHunt · #5 09-30-2019, 01:17 PM

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.

Simon Hitchen · #6 09-30-2019, 05:38 PM

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.

Simon Hitchen · #7 09-30-2019, 06:23 PM

Tim, thanks for pointing out Mike?s write up. Just what I needed.

mturnerb · #8 10-01-2019, 05:08 AM

I?d inspect plugs for fouling too just to be thorough.

JonJay · #9 10-01-2019, 08:49 AM

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.

lr172 · #10 10-01-2019, 09:01 AM

Quote:

Originally Posted by Simon Hitchen

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