Engine stumble
 

O-360 with dual EFII ignition with autowires/plugs, rebuilt 2009, first engine start Sept 2014 and 96 hours without any rough running etc last year. 10 hours since condition inspection, during the condition inspection I sent the Carb to Marvel Schebler and they enriched it slightly and replaced the bowl. First 6 hours after the condition everything ran as it should. I did get fuel at the airport before the issue, but my tanks sumped clean in the pre-flight and I had them double check their truck this morning.
I flew about 2.5 hours yesterday morning without any issues, 3rd leg of the day I slightly leaned in the climb about 120-150 ROP and leveled off at 13.5. I accelerated to cruise speed, CHT's were all 380 or less I left full throttle in and made a big pull to LOP (I only do LOP below 65% power). About minutes after setting 20-80 LOP the engine ran rough like it was ingesting water or had bad carb ice (too instantaneous for carb ice). It felt like a LONG time, data shows just over a minute of rough running. I swapped fuel tanks, turned on the boost pump with no change. When I went full rich with the mixture it bogged the engine more and I leaned it about where I thought it should be it eventually smoothed out.
I pulled the SD cards and looked at the data this morning, about 250 RPM and 10 knts IAS loss in the minute. Fuel flow went from 7.8 to 9.6. EGTS were all 1310-1330 before and dropped to 895-1070 with #2 being the lowest. It was long enough that CHT's even dropped around 30 degrees.

After it smoothed out I tried ROP and LOP and both ran fine, after running LOP for about 30 minutes I started descending. I left the throttle full in and mixture at 20-80 degrees LOP. At about 13.2 the engine ran rough again for about a minute with the same drop in EGT's/CHT's/RPM/Airspeed. Small mixture adjustments as well as large mixture adjustments didn't seem to make a difference. After it smoothed out it ran flawlessly through several
level offs and different mixture/power settings for the last 40 minutes of the flight.

After startup on the ramp for my last leg I had the same stumbling as I advanced the throttle through 1200 RPM to taxi up-hill. It wasn't very noticable, I felt it more than heard it. I was able to hear it after removing my headset. Switching ignitions behaved as I would expect with a slight loss of RPM but still rough running. I aggressively leaned it out and it smoothed out after about 45 seconds. I did an extended runup and everything ran fine, after a cautious takeoff the hour flight back home was uneventful.
The data on the ground showed the #2 EGT during the rough running was 500 degrees while the other 3 cylinders were 1250's.

Looking back over the data for the previous flights of the morning when everything ran normal. During a full power, Full rich climb #2 is my richest cylinder with EGT's 180-200 degrees cooler than the other cylinders. When LOP in cruise it is within 10-15 degrees of the other cylinders.

I'm looking for things to check when I pull the cowl, ideas?
Thanks,
Andy

Carb problem....something that made it go very rich. Stuck float valve, loose bowl, loose jet...? Perhaps an airbox obstruction.

[quote=crabandy;972045]
The data on the ground showed the #2 EGT during the rough running was 500 degrees while the other 3 cylinders were 1250's.


Maybe a sticking valve issue on cylinder #2.

I did just re-make all of the hard fuel lines, 2 of them after the fuel filter. Perhaps a chunk of something made it to the Carb, I'll probably start there.

I agree with Mike, this is a dead give away, it's telling you #2 cylinder quit running during the event, I don't think the carb will cause one cylinder to drop off. a sticking exhaust valve is a single point of failure where by changing the mixture, switching tanks or checking both mags/EIs will not clear up the rough running.

Could it also be related to ignition or spark plug on that one Cyl as another option?

So just got done doing some more trouble shooting. I fired the airplane up and it was rough running, it wasn't obvious on the G3X because I had my engine monitor on the checklist page. With the checklist displayed the engine parameters are truncated and only show hottest EGT/CHT. I switched to the main engine page and it was obvious the #2 was not firing correctly with a 500* EGT, exactly as seen on the engine data on the ramp the other day. I aggressively leaned it and #2 started running normal, again about a 1 minute event.
I did another extended run-up and cautious takeoff and short flight but couldn't reproduce the problem ROP/LOP. Landed, fueled up and the re-start/taxi back to the hangar was all normal.
Pulled the cowl and started an early oil change. Carb shows no signs of leakage on the outside or in the throat. I did speak to the Mark at Marvel Schebler (great resource) and besides some sort of leakage/debris carbs generally don't "fix themselves."
At this point I'm fairly certain it is an ignition/spark plug problem. I pulled the plugs on the #2 cylinder and I (not an expert) think they look fine, granted they were firing just fine 30 minutes ago. They are NGK BR8EIX 5044 auto plugs with adapters smeared with anti-seize.


I measured the spark plug resistance at 3800-3900 on both warm. Plug wires measures 800-1250 ohms with the longer wire measuring 1250.

Before I just put new plugs in it and fly it how do I rule out a sticky valve?

Just an FYI/observance here, I believe you should install the plug adapters, and leave them in the head, remove only the plugs.

What are the other plugs in the engine looking like, and testing for resistance?

The fact the problem comes and goes would support a sticking valve issue...................

Mike S,
I put the socket on the sparkplug and both come out, it's consistently been that way when I remove them. I finger tighten the adapter on the sparkplug and then screw/tighten that assembly to 30 ft/lbs via the sparkplug. All the plugs looked similar when I removed them, but I didn't put the ohm meter on them.

I do recall a small burble while switching ignitions during a runup on several occasions within the past 15 hours. With the carb jetted slightly richer now perhaps it's a plug/fouling issue?