Fouled Plug Identification Ideas

[ltdriser]

During run-up this morning I swapped to the L mag and got a pretty noticeable stumble and ~250rpm drop. R mag was working just fine, smooth and 100rpm drop. 
 

I tried to clear the L mag by leaning out on the ground, set RPM to 2000 and let her sing. After a 2-3 15 second runs and mag checks I gave up. No avail, back to the hangar we go. 
 

I plan to pull cowling, find the suspect plug and give it a good inspection/cleaning. My thought was to run the plane on the bad mag for a minute or two, remove the cowling at pit crew speed, then identify the cold exhaust. Narrowing down the cylinder is first order of business, unfortunately I don’t have a fancy engine monitor.  Is this a technique any one has done before? Maybe there is a better technique? 
 

Tips and Tricks appreciated!

[RLCarter]

Remove the top cowl and side panels before you run it, but unless it’s a dead miss it won’t be too easy with out something to read temp change between cylinders 

[A64Pilot]

You can do as you suggest with an IR thermometer, but it’s my opinion if ones fouled, why not clean them all?

To use the IR temp gun just taxi back on the “bad” mag and shut down the suspect cylinder and exhaust should have cooled enough by then to be obvious.

Spit on your finger and touching the exhaust for just a sec will work too.

Taxiing leaned out will often prevent this, but if you lean to taxi, lean the snot out of it. lean it so much that it won’t take throttle above 1500 or so, that way you can’t take off leaned out, at taxi power you can’t hurt anything at all with mixture.

Edited August 14, 2021 by A64Pilot

[Hank]

Touch each exhaust pipe with a paint marker ~1" from the cylinder. Three will burn off in a 1-minute run, the cold cylinder won't. Pull both plugs in the cool cylinder, clean them and swap them top to bottom to put each plug on the other magneto. Run up again. 

If roughness moves to the other cylinder, you know which plug to replace. Otherwise you know the plug is good, so check the obvious like worn / cracked plug lead, good fuel delivery, etc. If nothing, at least you've opened up the plane and completed initial diagnostics for your A&P.

This is where I am right now, too . . . 250 RPM drop on right magneto, Cylinder #1 cold, plugs good.

When I suspect a fouled plug, I advance to 2000, lean to peak and sit there for 60 seconds. Why do you only go for 15 seconds?

[ltdriser]

Thanks for the tips guys!

I only ran 15 seconds because in the past that’s all I’ve ever needed. 

[Hank]

I've always run a minute because that’s what I was taught as a student . . .

[Shiny moose]

You should just Remove all, examine all, clean all, ohm all, gap all. If this is an ongoing problem then find the cause, bad plug, intake leak, to rich mixture, oil fouled, wires old, mag issue, etc. 

[Prior owner]

16 hours ago, Hank said:

Touch each exhaust pipe with a paint marker ~1" from the cylinder. Three will burn off in a 1-minute run, the cold cylinder won't. Pull both plugs in the cool cylinder, clean them and swap them top to bottom to put each plug on the other magneto. Run up again. 

If roughness moves to the other cylinder, you know which plug to replace. Otherwise you know the plug is good, so check the obvious like worn / cracked plug lead, good fuel delivery, etc. If nothing, at least you've opened up the plane and completed initial diagnostics for your A&P.

This is where I am right now, too . . . 250 RPM drop on right magneto, Cylinder #1 cold, plugs good.

When I suspect a fouled plug, I advance to 2000, lean to peak and sit there for 60 seconds. Why do you only go for 15 seconds?

High tension lead testing is next, perhaps? Seems like it’s on the high tension side- maybe the magneto cap is bad?

[A64Pilot]

Thsi SB may be of some interest as it addresses plug fouling and it’s prevention.

https://www.lycoming.com/sites/default/files/Spark Plug Fouling.pdf

Notice the shutdown procedure. if you follow it and get into a habit of a quick mag check on shut down it’s likely you will never again be faced with a fouled plug on run up before takeoff, you’ll find it on shut down and can then get to it after the engine is cold at your leisure.

The SB doesn’t recommend a mag check just prior to shutdown, that’s my addition, but why not check to see if you have a fouled plug?

[ltdriser]

Wet plug front bottom passenger side with some chunks of lead in it. Cleaned it out and she runs much better now.  We did find a spark wire chafing through its braided steel sheath during my investigation on pilot side top front cylinder plug. So that will need replaced. 

[DXB]

20 hours ago, ltdriser said:

Wet plug front bottom passenger side with some chunks of lead in it. Cleaned it out and she runs much better now.  We did find a spark wire chafing through its braided steel sheath during my investigation on pilot side top front cylinder plug. So that will need replaced. 

BTW If this is a carb'd plane, it needs aggressive leaning on the ground right after startup to keep from fouling.  I learned the hard way when I first got my C - which also didn't have an engine monitor at the time so I got some practice finding the fouled plug quickly by intuition .

 It's more often a bottom plug, so that narrows down to four plugs to check first.  The routing of the harness from that mag should be to top on one side and bottom on the other, so that gives you two plugs to check first.  And in my experience it's the same offending bottom plug over and over - on the richest and/or oiliest of the bottom cylinders.  An idle mixture that's too rich may also contribute to the issue, so it's worth looking at that.

 

 

[jaylw314]

On 8/15/2021 at 7:08 AM, A64Pilot said:

Thsi SB may be of some interest as it addresses plug fouling and it’s prevention.

https://www.lycoming.com/sites/default/files/Spark Plug Fouling.pdf

Notice the shutdown procedure. if you follow it and get into a habit of a quick mag check on shut down it’s likely you will never again be faced with a fouled plug on run up before takeoff, you’ll find it on shut down and can then get to it after the engine is cold at your leisure.

The SB doesn’t recommend a mag check just prior to shutdown, that’s my addition, but why not check to see if you have a fouled plug?

That's what I was taught to do, it seemed whenever I forgot or skipped it, I'd find a fouled plug the next flight and have to burn it off.  Since leaning aggressively during taxi, I have not had to redo a mag check in the last 4 years, so I've stopped bothering, but if you're not in the habit of leaning during taxi, it's a must.

[A64Pilot]

Just if your lean for taxi, lean so aggressively the engine won’t run above 1500 or so without missing heavily.

People have taken off leaned out and been killed from an engine that won’t develop enough power to climb, but if you lean aggressively, that can’t happen.

[steingar]

I'm fairly certain that this was brought on by a lot of flying at low speed and high mixture settings (IFR approaches).  Aircraft gets leaned on the ground and in the air best we can without an EGT gauge.  Aircraft goes back to its mechanic next week.  Could be the bad wire we found, could be the magneto timing, could be the bad wire leads to a bad plug.  I'll let someone with way more experience than me sort it out.  Needs new spark plug wires anyway, and I can't do that.