Bad Mag Check

BaronPete

BaronPete

Jaw Harp Troubadour
Hey guys, was looking for some insight / advice for a bad mag check I did today in a C150J.

I did some searching and reading, but while some posts got close, I didn't feel as though any specifically answered my situation.

I just left the airport where I was supposed to be doing some final PPL checkride prep work. Anyway, I taxied out and as I'm doing my mag check, my left mag showed a barely perceptible drop in RPM. Now, I switch over and check my right mag and the engine begins running rough with the RPM bouncing around between a 200-100rpm drop.

This has happened before and it was demonstrated to me how to burn off lead accumulations on spark plugs, but I was unable to fix the problem. I taxied back in and called my instructor out (he was aggravated) and he leaned the mixture a bit, did the runup, waited a bit, then rapidly went through the mag check and all seemed well.

I taxied out again, and decided to do another runup. Same problem. Very little drop on left, rough engine and large drops on right. I radioed in again and he told me just to taxi in and tie it down.

I'll admit to not leaning the mixture while taxiing :(, but I feel pretty bummed about not knowing my stuff. He said he would test it out later and likely break the plane down and replace/check out a bunch of stuff, but I kinda got the feeling from him that I just didn't know what the hell was going on.

Sorry for the long post, but what could have been the problem? The left mag moved SOME, so a loose p-lead doesn't seem to be the case. He also said you shouldn't check a single mag for more than 3 seconds, as that can cause roughness. :confused: I'm unsure how long I actually left the engine on just the right mag that was under performing, but it seemed like when he did HIS mag check it was WHAM WHAM with almost no time inbetween.
 
BaronPete said:
He also said you should check a single mag for more than 3 seconds, as that can cause roughness. :confused: I'm unsure how long I actually left the engine on just the right mag that was under performing, but it seemed like when he did HIS mag check it was WHAM WHAM with almost no time inbetween.
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I wonder where he got that 3 second number from. I've never heard that.
 
meritflyer said:
Sounds like a fouled plug.
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That is what I'm hoping..It just frustrated me that I found the problem, brought it in, wasn't able to reproduce it during his magcheck, yet as soon as I went back out (3 minutes later) the same thing was going on.

And as far as where he got the 3 second bit, I'm not sure. My instructor is an old military aviator. He's a great instructor, but tends to be a bit on the ornery side a great deal of the time.
 
mikecweb said:
I wonder where he got that 3 second number from. I've never heard that.
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Also, I just realized I omitted the "n't" from should. He said the mag check shouldn't be longer than about 3 seconds as it would eventually lead to roughness.
 
I wouldn't feel bad if I were you, if you had any reason to feel unsafe about going up in an aircraft then don't. I'd say good on you for making your own decision regardless of what someone else said. If you're the PIC it's your call. I remember a flight where I didn't follow my gut feeling and let someone else influence a decision which should have been mine and mine alone to make, and while nothing bad ever happened, I regret not putting my foot down and telling them "You know what, I'm the PIC and don't think this is a good idea."
 
JordanD said:
I wouldn't feel bad if I were you, if you had any reason to feel unsafe about going up in an aircraft then don't. I'd say good on you for making your own decision regardless of what someone else said. If you're the PIC it's your call. I remember a flight where I didn't follow my gut feeling and let someone else influence a decision which should have been mine and mine alone to make, and while nothing bad ever happened, I regret not putting my foot down and telling them "You know what, I'm the PIC and don't think this is a good idea."
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Thanks for the boost. As I was sitting there I was thinking: "Well, I came out to practice engine failures to accurate landings...since I'm rusty on those I'd rather not be trying to do a real one, even if going back in means an ass chewing." :o
 
Rough mag checks aren't uncommon in the 150 with the Continental O-200. Depending on the severity of the drop, I'll usually increase the RPM to about 2100RPM, lean out the mixture as you would during cruise (until RPM drop), and leave it there for about 15-30 seconds. Mixture forward, 1700RPM, mag check again. If it doesnt clear, at that point I'd usually go full throttle, lean it again, and leave it there for a good amount of time 45-60sec.

Typically I don't go full throttle in an attempt to save wear and tear on the prop. But it's there if you need to. One other thing you might try to do is perform a run-up with a lean mixture just to see if that works.

After the 2nd or 3rd attempt at a mag check though, I'm bringing it back to the ramp.
 
Crism said:
Rough mag checks aren't uncommon in the 150 with the Continental O-200. Depending on the severity of the drop, I'll usually increase the RPM to about 2100RPM, lean out the mixture as you would during cruise (until RPM drop), and leave it there for about 15-30 seconds. Mixture forward, 1700RPM, mag check again. If it doesnt clear, at that point I'd usually go full throttle, lean it again, and leave it there for a good amount of time 45-60sec.

Typically I don't go full throttle in an attempt to save wear and tear on the prop. But it's there if you need to. One other thing you might try to do is perform a run-up with a lean mixture just to see if that works.

After the 2nd or 3rd attempt at a mag check though, I'm bringing it back to the ramp.
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That seems like pretty good advice to me.

Definitely try to remember to lean the mixture when taxiing. Remember, the only way to run too lean when taxiing is if the engine quits when you advance the throttle to get moving.
 
Roger said:
That seems like pretty good advice to me.

Definitely try to remember to lean the mixture when taxiing. Remember, the only way to run too lean when taxiing is if the engine quits when you advance the throttle to get moving.
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We're huge proprietors of lean mixture down in Daytona. Typical setup is engine start, 1000rpm, check everything in the green (or heading towards it), 1200RPM, lean until decrease in RPM, leave the mixture as-is for a smooth running engine, 1000RPM.

That'd be the official way to do it. 90% of the time it's about a 1in to 1 1/2in from full forward so I pull it out about that far and call it a day. With student getting in the habit of doing it, I'll use the official method but for those with common sense who have done it before, I'll keep it quick.
 
Update:

Instructor just called me back. He went ahead and flew it later on after I left and even soloed a student today with no issues and was unable to recreate the problem I had. That really irritates me, but thanks for all the advice guys!
 
Crism said:
We're huge proprietors of lean mixture down in Daytona. Typical setup is engine start, 1000rpm, check everything in the green (or heading towards it), 1200RPM, lean until decrease in RPM, leave the mixture as-is for a smooth running engine, 1000RPM.

That'd be the official way to do it. 90% of the time it's about a 1in to 1 1/2in from full forward so I pull it out about that far and call it a day. With student getting in the habit of doing it, I'll use the official method but for those with common sense who have done it before, I'll keep it quick.
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It seems that hot/humid weather tends to foul plugs. I taught in East Texas and we were religious about leaning the mixture. I was a bit more fanatical than many people...I'd fly with other people's students and they'd do a twist or 2. I typically did what you do: 1-1.5 inches out (since you work for ERAU I'm guessing you rock the C172R with IO-360-L2A [an awesome motor in my extremely limited experience]). If the motor tried to quit when I started taxiing out of my parking spot, I'd richen it a bit.
 
Roger said:
It seems that hot/humid weather tends to foul plugs. I taught in East Texas and we were religious about leaning the mixture. I was a bit more fanatical than many people...I'd fly with other people's students and they'd do a twist or 2. I typically did what you do: 1-1.5 inches out (since you work for ERAU I'm guessing you rock the C172R with IO-360-L2A [an awesome motor in my extremely limited experience]). If the motor tried to quit when I started taxiing out of my parking spot, I'd richen it a bit.
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You got it. C172S IO-360-L2A and the Conti O-200 on the 150s. Those 172s take a beating, and I've never really had a problem with them other than an occasional rough mag. The O-200s are a different story. I'm in the process of getting times on the engines, but they're pretty darn used and abused.

We've got 2 types of mixtures here. The older airplanes have the mixture that you can roll out 6 times and it's perfect. The newer ones have the mixture that rolls probably 3x the amount to yield the same result. Basically, same mixture, one just has a lot more thread to it and rolls out a lot slower. Kids don't realize theres a difference, and it's just sad to see students who add power, get the engine sputtering on the taxi (up the hill out the ramp), and they don't know what to do.
 
Crism said:
Rough mag checks aren't uncommon in the 150 with the Continental O-200. Depending on the severity of the drop, I'll usually increase the RPM to about 2100RPM, lean out the mixture as you would during cruise (until RPM drop), and leave it there for about 15-30 seconds. Mixture forward, 1700RPM, mag check again. If it doesnt clear, at that point I'd usually go full throttle, lean it again, and leave it there for a good amount of time 45-60sec.

Typically I don't go full throttle in an attempt to save wear and tear on the prop. But it's there if you need to. One other thing you might try to do is perform a run-up with a lean mixture just to see if that works.

After the 2nd or 3rd attempt at a mag check though, I'm bringing it back to the ramp.
Click to expand...

Also, another question regarding this: When performing the steps you listed above, would I leave the ignition switch turned solely to the magneto that was giving me trouble? Or would I switch back to both to complete these steps and then go back and do a regular mag check?
 
Crism said:
You got it. C172S IO-360-L2A and the Conti O-200 on the 150s. Those 172s take a beating, and I've never really had a problem with them other than an occasional rough mag. The O-200s are a different story. I'm in the process of getting times on the engines, but they're pretty darn used and abused.

We've got 2 types of mixtures here. The older airplanes have the mixture that you can roll out 6 times and it's perfect. The newer ones have the mixture that rolls probably 3x the amount to yield the same result. Basically, same mixture, one just has a lot more thread to it and rolls out a lot slower. Kids don't realize theres a difference, and it's just sad to see students who add power, get the engine sputtering on the taxi (up the hill out the ramp), and they don't know what to do.
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That's interesting that Cessna changed the mixture controls. As for the IO-360-L2A...we had a fleet of 7 of them. We ran them anywhere from 200-400 hours past TBO. In our training environment, they were subject to every kind of abuse imaginable. As far as I know, in that time span we never even had to change a cylinder. Comparing that with some of the engines I'm seeing now in the mx shop...wow.
 
BaronPete said:
Also, another question regarding this: When performing the steps you listed above, would I leave the ignition switch turned solely to the magneto that was giving me trouble? Or would I switch back to both to complete these steps and then go back and do a regular mag check?
Click to expand...
Nope, run on both. Think about it...you're trying to burn off the fouling. If you are running only on the rough mag, the cylinder with the bad plug isn't firing and therefore isn't getting hot, preventing the fouling from burning off.
 
Roger said:
It seems that hot/humid weather tends to foul plugs. I taught in East Texas and we were religious about leaning the mixture. I was a bit more fanatical than many people...I'd fly with other people's students and they'd do a twist or 2. I typically did what you do: 1-1.5 inches out (since you work for ERAU I'm guessing you rock the C172R with IO-360-L2A [an awesome motor in my extremely limited experience]). If the motor tried to quit when I started taxiing out of my parking spot, I'd richen it a bit.
Click to expand...


same here...
 
Fouled plugs on o-200's are all to common. about 25% of my flights in citabrias with the o-200 have a fouled plug, sometimes they clear sometimes they dont.

I start the engine leaned about an inch to 1.5 inches. I leave it there at no time do I go full rich. That as always seemed to work for me.
 
BaronPete said:
Also, another question regarding this: When performing the steps you listed above, would I leave the ignition switch turned solely to the magneto that was giving me trouble? Or would I switch back to both to complete these steps and then go back and do a regular mag check?
Click to expand...

Ya switch to both. When I need to burn off fouling I go to 2000rpm's on both mags, lean 50 degrees lean of peak and run it for 30 seconds. Then back down to 1700 or 1800 and recheck.
 
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