Fair warning! Do not read if you don't like mysteries.

[rcfaubion]

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.

[Mike S]

Originally Posted by rcfaubion

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

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.

[Don Jones]

Robert,
If you will make the wind stop, I will come out there and help you figure it out.

[aerhed]

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.

[aerhed]

Also, like Mike said. Make sure there's not a potato in your exhaust.

[David-aviator]

Originally Posted by rcfaubion

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.

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.

[az_gila]

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...

[David Z]

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.

[170 driver]

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.

[John Clark]

Originally Posted by 170 driver

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.

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