Burning out a fouled spark plug

[RobBurson]

Here is the hypothetical scenario. Mag check 250 rpm drop on right mag, left fine. Try 3 times to burn it clean, no go.

What do you do?

A: Fly anyway
B: Pull the plug
C: Fly for 5-10 minutes land and do another mag check

Can you do a mag check in flight at any RPM?

Inquiring minds want to know.

Rob

[Maule 9V]

Choice D- Change lower plugs to fine wire.

[Norm]

It's amazing to me how bad an engine runs when only one plug is fouled. On my IO-540 260 HP I have fouled a plug and had success running the engine at 2000 RPM and lean the *heck* out of it and cleared a fouled plug that way. Otherwise I'd pull the plug and clean it ... unless you're taking off on a 20,000' long runway.

I can't believe how agressive I have to be with my engine on keeping it leaned out in taxi and descent otherwise I'll foul up -- and that's with all fine wire plugs on the bottom.

[kakkenmc]

Norm,

I've never leaned in descent. In fact, I generally gradually enrichen. Share any details?

Kennon

[Duane]

Norm,

Those are some of the most incredible pics I've seen.. very touching tribute vid.. looks like Shaun had an incredible life.. wish I had half his adventures. My condolances on your loss

[Green Hornet]

Here is the hypothetical scenario. Mag check 250 rpm drop on right mag, left fine. Try 3 times to burn it clean, no go.

What do you do?

A: Fly anyway
B: Pull the plug
C: Fly for 5-10 minutes land and do another mag check

Can you do a mag check in flight at any RPM?

Inquiring minds want to know.

Rob

How do you know it is one plug?

I was with an instructor who was determined to clear the fouled plug like he has done so many times with his Citabria. Since that was his airplane I went along with the program even though I had my doubts.
When a similar thing happened in my airplane after the second take off and the engine was still running rough I opted to call my PIC privilege and call it a day. The mechanic found three (3) bad plugs that he replaced. One mechanic scratched his head and thought it was an engine problem. It was the engine guy who said lets check the plugs and found the culprits.
It does not take that much time to check the plugs I was taught how to do it in about 90 min. check, clean and regap! Since you own your own airplane you know whether this is unusual but in my book you only have one engine (SEL). Better safe than sorry!!!!

[donknee]

I have had a bad run-up within 3 hours after annual, freshly cleaned and gapped plugs. I suspected a fouled plug...but which one? With the cowl off I ran the engine for about 5 minutes, shut down, took an infrared heat thermometer and measured the temp of each plug and it was obvious which one was fouled, not firing and colder. Pulled the plug, it was wet from un burnt fuel. Put new plug in and problem solved. I think the plugs went bad, since they didn't look fouled. Possibly dropped when cleaned? Cracked the insulator? I don't think they had over 50 hours since new. I carry a spare plug and tools now.

[Norm]

I was still getting used to my M7 when I was descending from 7500' into Moose Creek, ID, elevation 2450' and I enriched the mixture as I've done with other planes and I fouled a plug or plugs. I got it (or them) to fire by doing a long runup with a super lean mixture. I made a mental note to keep the mixture near peak EGT on descent which, for about 4000' feet of descent, usually means leave it pretty much where it was in cruise. I always have two spare plugs in my tool kit.

I've never flown an engine like my IO-540 that wants to foul so much. I'm sure it would even foul more if it weren't for the fine wires on the bottom. I lean very aggressively during idle and taxiing -- just enough mix to keep the engine running smoothly -- and I think this really helps at a time when the cyclinders are cool and are maybe more prone to fouling. During idle and taxi if you look at my mix control you'd think it was at idle cutoff.

Thanks, Duane, for your condolances. Time heals all wounds ... except one like this.

[a64pilot]

Rob,
Yes you can do a mag check in flight. I didn't say that I could find a procedure in writing saying it was OK, but the engine isn't aware it's flying or on the ground. Try it, you will lose a little power, but watch what happens to your EGT's, it's interesting. If you haven't taken off on one mag, you will eventually. Stay at cruise power and RPM, no need to reduce power drastically to do a mag check in flight. In fact in really nasty runway environments, it may be smarter to do your mag check in flight in the pattern, and fore-go your prop cycle.
If you have a fouled or bad plug it's easy to find exactly which one with your E.I., Just simply run it on the "bad" mag for a short period and the "bad" cylinder will show a noticeable drop in EGT.
Fine wires are the ticket, I run all 12 as fine wires. I see a little bit lower fuel flow for peak with fine wires, that and the fact they are supposed to last the life of the engine will make them more than pay for themselves in the long run.
Rob, running car gas, you should be less likely to foul plugs as us that run 100LL. I would be a little more suspect of a rough engine running car gas than I would with one running 100LL. If you have lead fouled a plug, you probably will not be able to "burn" it off, usually you have to clean a lead fouled plug.
Guy's with fine wire plugs, do not get near them with a bead blaster. All you should really have to do with fine wires is to remove the clinkers with a sharp instrument, check gap and rotate them every 100 hours.

[a64pilot]

Norm,

I've never leaned in descent. In fact, I generally gradually enrichen. Share any details?

Kennon

Kennon,
You are correct, except for the folks that run LOP. LOP you can just leave the mixture alone as the more you descend, the leaner it get's and therefore the cooler it runs, up until the point it get's rough of course.
My technique is to reduce power to keep the manifold pressure the same as in cruise and slowly enrichen the mixture to keep the EGT the same as in cruise, and let the speed build. Usually it's around 10 kts or so for a 500FPM descent. Just my technique. As I get closer to my landing point, I continue reducing power until I'm in the pattern. A rule of thumb I have heard is to try to keep power reductions to 1" per minute if possible in the descent to landing phase to avoid cooling things too quickly.
I've also heard that if the CYL head temp is below 400, you can do anything you want to with the throttle, you can't hurt it. I'm not sure I believe that as my CYL head temps are never over 400, ever.

[Norm]

Norm,

I've never leaned in descent. In fact, I generally gradually enrichen. Share any details?

Kennon

I agree with a64pilot. But, if you descend at peak EGT (whatever the power setting) you should keep the plugs from fouling. Then, on short final I richen things up a little in case of a go-around. Then, when clear of the active and taxing to the hanger I lean like *mad*, but just rich enough to keep the engine running smooth.

[a64pilot]

If you lean on the ground, make sure it's so lean you can't advance the throttle much before it starts it's lean stumble, or you will eventually take off leaned out

[Norm]

Yep. And when I'm leaned out for taxi I have to enrich it some before it will run smoothly for a runup, too. It seems like I'm jockeying the mixture a lot but I don't get fouled plugs doing it this way.

[kakkenmc]

If you lean on the ground, make sure it's so lean you can't advance the throttle much before it starts it's lean stumble, or you will eventually take off leaned out

How quickly will you fry your too lean engine? Is it a matter of overheating or do other things happen?

[a64pilot]

How quickly will you fry your too lean engine? Is it a matter of overheating or do other things happen?

At low manifold pressures, say below 20", you can do anything you want to with the mixture control and not hurt anything, unless full rich is too rich, then excess fuel washes oil off of the cylinder wall, dilutes the oil and fouls plugs and builds up lead in the oil. Now I'm talking stupid, black smoke chugging rich, so don't worry about staying full rich on the ground. BTW, your mixture is correct if from full rich you gradually lean and get a 50 RPM rise before it cut's out at idle.
Lycoming allows operation at peak at 75% power and below. You can't get hotter than peak The problem comes above 75% power if leaned. If in doubt, always err to the rich side, you won't hurt anything. Too lean, at too high a power setting, and best case you spend $35,000 on a new engine, worst case and you are in a forced landing, with a hatched engine.