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rcfaubion

Member
Sorry for the length of the post but I wanted to get all the info out at the start.

Recently my engine developed a slight roughness and change of tone in flight along with a slight loss of power. *Time for annual condition inspection anyway and when I pulled the plugs the mystery began. *Both plugs on #3 cyl were very sooty, not oily.

History. *I am not the builder. *This aircraft (RV-4) was built in 2001 and had 300 hrs TT when I purchased it, I have put 200 hours on it in a little over a year. * *Uses almost no oil (a little over a qt between oil change). Plugs burn very clean and very uniform, compressions are great, all in all a good engine.

Setup. *Aero Sport Power Lycoming O-320 D2A, Marvel/Precision MA-4SPA carb., Catto prop, Slick Mags, Elec boost pump (no primer lines to any cylinder), 500 hours since Aero Sport Power o/h. *4 pipe Vetterman exhaust. *Spark plugs Champion REM38E, single cylinder (#4) egt/cht (I know I need a multi channel ems).

Performed these tests/checks/inspections to no avail! *Found no problems and the sooty plug problem remained. *Plugs soot up with just a short ground run up, mag drop 50 rpm on each mag. *Performed a run up after each of these tests.

1 bench tested, gapped, and rotated plugs.
2 tested ignition harnesses (also tried different harness)
3 compression #3 76/80 (others were 75-78/80)
4 dynamic compression (automotive) 150 on all
5 removed mags and did 500 hr inspection/cleaning, no indications of any problem. Remember a mag drop of 50 on both mags.
6 checked for induction leaks. *None
7 checked for sticky exhaust valve, broken valve spring, bad lifter. *None
8 removed #3 jug, checked valves, seats, piston, cylinder, head, cam, lifters again, rings, oil control ring, push rods, rockers, valve travel. *Nominal
9 pulled a vacuum on the intake to check for possible crack in sump
10 did a run up using only the left mag. *#3 plugs sooty, others very clean for only running on one mag!
11 pressurized carb with boost pump, no leaks or seeps.
12 don't know what else to do! *Several knowledgeable A&Ps at my airport are stumped on this problem.
13 have not tried a different carburetor, yet.

All work done by or with the supervision of my A&P.

Thanks in advance for your consideration on this baffling problem.
 
rcfaubion said:
Both plugs on #3 cyl were very sooty, not oily.
Click to expand...

Sooty, but not oily----------soot is a byproduct of combustion, where the fuel is not totally consumed. Fuel in this case is not necessary avgas, it could in fact be small amount of oil.

An "oily" plug is caused by more oil getting into the combustion chamber than can be burned.

Another cause is the fuel mixture being overly rich----not too probable in a single cylinder. Off the top of my head, I can think of only one scenario that would cause that, and that is an partial obstruction in the exhaust for that one cyl, that somehow is causing the cyl to become overly rich-------????

Start by doing a compression check with a leak down tester, like is normally used in aircraft.

And, check out your exhaust system.
 
This looks tough on the surface, but I'm gonna guess surface (cylinder / rings) this time. I know you looked at them, but I would hone it & change rings. That's provided all the other checks you did are valid.
Also, do what Mike said and do a diff compression first. You may have a bad zone or spot mid-travel or a valve leak even.
 
rcfaubion said:
Sorry for the length of the post but I wanted to get all the info out at the start.

*Both plugs on #3 cyl were very sooty, not oily.

Plugs burn very clean and very uniform, compressions are great, all in all a good engine.


Thanks in advance for your consideration on this baffling problem.
Click to expand...

There is a slight contradiction in the report. #3 cyl very sooty, not oily.....plugs burn very clean and uniform....

Could it be on the flight where a slight loss of power was notice, one set of plugs sooted up due to not leaning the engine that day?

Just a shot in the dark. None of this makes any sense. Everything has been checked normal, maybe the engine is normal.

I flew earlier this week in 60F weather, today it was 84F. The engine did lose some power and sounded a little different.

But it is your airplane and if it sounds different and acts different, something is different. Lycomings have a way of sending signals when something isn't right. But you've checked everything there is to check so you're correct about one thing, it is a mystery.
 
Since Las Cruces is 4400 ft altitude and would have a density alttude of 5000 ft with an air temp of around 50 F, could you just be running too rich now spring is here?

Try a full power run up and lean to maximum rpm just like Lycoming says for a DA over 5000 ft. and then check the plugs.

The multi-channel EMS would help...:)
 
Intake leak?

You might have an intake leak that's leaning out the other 3 cyliders.
Has your throttle closed, minimum idle speed changed at all? If you have an intake leak you'd be sucking some air at idle, and it would idle slightly faster.
Just an idea.
 
If your primer line is going to that cylinder, it is easy to disconnect and plug off the system to see if that is the cause.
That was causing my 320 to run rough, primer was leaking into the cylinders. It made both the cylinders that had primer lines in them run rich especially when the throttle was pulled back after a climb.
 
Nope

170 driver said:
If your primer line is going to that cylinder, it is easy to disconnect and plug off the system to see if that is the cause.
That was causing my 320 to run rough, primer was leaking into the cylinders. It made both the cylinders that had primer lines in them run rich especially when the throttle was pulled back after a climb.
Click to expand...

My first thought too, but the original post says "no primer lines to any cylinder"

John Clark ATP, CFI
FAA FAAST Team Member
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA
 
I just read an article in either EAA or AOPA about a Bonanza owner that had something very similar to this. Turned out to be an exhaust valve with too much wobble that ended up requiring a full tear down. He narrowly avoided catastrophic failure.


Could be totally unrelated, but the symptoms sound very similar.
 
Keep the ideas coming

Don, I'm doing my best on the wind, without much luck.
1. No potato in the exhaust.
2. Diff comp 76/80, see item 3 in original post
3. Very familiar with DA issues, have operate at high and hot locations most of my life and lean aggressively. (very sooty plugs on just one cylinder after a short run up)
4. until this problem the plugs burned clean and uniform. This condition came suddenly within the last few hours. Had the plugs out for another reason within the last ten hours--clean
5. No primer lines to any cylinder, Elec boost pump.
6. Valve guides seemed nominal.

Thanks for the ideas and keep scratching.
 
rcfaubion said:
Gil, my A/P used a standard diff comp tester. Not sure I know where you are going.
Click to expand...

Lycoming compression numbers are based on a 0.040 orifice. Some compression testers use a 0.060 orifice which has over two times the area.

If the wrong tester orifice was used your "good" compression readings would actually be quite poor....:(
 
Robert,
Short story here and one more thinig to look at. I had a problem a few years ago coming back from LOE. Airplane would run fine with good mag checks on the ground. Any time I got above about 7000' MSL the engine would start running rough. I isolated the mags at altitude and determined I had a problem on one side. I took apart the offending mag after not being able to find anything else wrong and found a cracked distributor block. You couldnt see it with the naked eye, but with a magnifying glass it was visible. It would only open up and cause problems in the air. If you dont find anything else you may want to take a closer look at that.
Ryan
 
I don't imagine, it would be too difficult to move the CHT, EGT probe to your #3 Cyl and get a temp reading and compare with other cyl or past. Also as it was suggested, would check the intake for any leak/obstructions? I don't know if it is possible to change this part with a temp one just to test or not.

Good luck
 
I like these kind of mysteries. One question I did not see asked was whether your oil consumption had gone up. EGTs and even air/fuel ratios for each cylinder would be real nice to have about now, but short of that, the only thing I can think of simple (?!) to try would be to completely swap 2 cylinders.
 
Issue resolved

The issue turned out to be a bad exhaust hydraulic lifter assembly on cylinder three. It had been visually inspected and looked ok. No debris no scoring etc... Finally just replaced it with another and installed a slightly shorter push tube as the valve lash was a bit tight on that valve(which may have caused the failure to begin with).
 
Great News.

lcnmrv8r said:
The issue turned out to be a bad exhaust hydraulic lifter assembly on cylinder three.
Click to expand...

Thanks for reporting back on this.

As I suspected, exhaust flow issue on that cyl------but I sure did not think of a bad lifter.

Learn something new all the time dont we???
 
I can see Robert's smile from here! Nice work Sherlock and Watson (Robert and Seth)! Glad you'll be back in action! Wenatchee Race, anyone?

Just to clarify for my numbskull brain, the bad lifter meant the ex valve in that cylinder was not fully opening, so you had bad scavenging, thus the soot? Did that lead to fouled plugs and made it even worse? Just trying to understand the dynamics and build the database of engine knowledge.

Thanks for closing the loop on this...was just wondering how it was going!

Cheers,
Bob
 
lifter pump up

rvmills said:
Just to clarify for my numbskull brain, the bad lifter meant the ex valve in that cylinder was not fully opening, so you had bad scavenging, thus the soot?
Click to expand...

More of a lifter pump up issue where the valve was being held open. Sooting up the plugs in that cylinder.
 
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