Bad mag drop during run-up proper procedure

RyanB

RyanB

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To clarify what I believe is correct, if during your run up you raise power to 1700 rpm and do your mag check, turn to left mag good drop, back to both, then right mag and you get a bad drop or the engine runs rough. First thing you do is leave it on that mag so the right mag, then you lean the mixture as far out as you can get without engine quitting and leave it like that for a few seconds to get engine good and hot to burn off the carbon, then full rich and turn key back to both and then try the mag again. If this is wrong please clarify the correct procedure. Thank you!
 
You need to be running it on both mags to get it to heat up and burn off the carbon.
 
yep, you need fire to burn off the crap. If the plug in that cylinder is totally fouled no amount of leaning will make that cylinder fire. You gotta use that second plug that's still good to clean the fouled one.
 
And it takes quite a lot longer than a few seconds.

I did one a week ago that took 2 1/2 minutes.

You could see the spark come back using its engine analyzer.
 
Hmm. Both instructors I have time with indicated that I should leave it on the rough mag when trying to burn off the fouling. So I guess I'm in the same boat as the OP: been doing it wrong?

Ahhh, the joys of being a low-time pilot. :D
 
asechrest said:
Hmm. Both instructors I have time with indicated that I should leave it on the rough mag when trying to burn off the fouling. So I guess I'm in the same boat as the OP: been doing it wrong?

Ahhh, the joys of being a low-time pilot. :D
Click to expand...

If you do that, the cylinder is not firing.

With an engine analyzer, you see no EGT reading for that cylinder.

I can't imagine how you could possibly burn that off without the other mag giving you some heat.
 
MAKG1 said:
If you do that, the cylinder is not firing.

With an engine analyzer, you see no EGT reading for that cylinder.

I can't imagine how you could possibly burn that off without the other mag giving you some heat.
Click to expand...

The whole concept is to generate as much heat as possible - you're doing this under cruise power, so you can run it at max pwr mixture and not hurt the motor (you're below 75%)...on one mag you are not getting near the heat, and think about this, you could now be fouling the other spark plug that isn't firing because it is no longer burning anything off...
 
MAKG1 said:
If you do that, the cylinder is not firing.

With an engine analyzer, you see no EGT reading for that cylinder.

I can't imagine how you could possibly burn that off without the other mag giving you some heat.
Click to expand...

engine analyzer is great. helped me to find a bad plug when I had an rpm drop issue during mag check.
 
MAKG1 said:
If you do that, the cylinder is not firing.

With an engine analyzer, you see no EGT reading for that cylinder.

I can't imagine how you could possibly burn that off without the other mag giving you some heat.
Click to expand...

I never had plug fouling issues until my flight school rolled in a new rule that we were no longer allowed to lean for taxi operations. I never understood why they did that. The only thing I could think of is that a student had failed to go full rich for takeoff, and that scared the school into implementing the "no lean for taxi" rule. (Now, as a member of a club, I lean aggressively for taxi.)

Instruction from my CFI went thusly: switch to rough mag, elevate RPMs a bit (2k?), lean it out until stumbling then enrichen slightly and run it like that for a bit. Switch to both and then do mag check again. :dunno:

So now I'm learning differently. I should be doing a similar procedure but on both mags. Apologies to the OP for adding my own questions, but I think they help answer his as well.
 
I was taught:
Mags both
Full mixture
Full static rpm
Lean peak rpm
Wait 60 seconds
Full mix
Throttle roughly 1800
Check again

Repeat if necessary. If no improvement, return to parking.
 
asechrest said:
I never had plug fouling issues until my flight school rolled in a new rule that we were no longer allowed to lean for taxi operations. I never understood why they did that. The only thing I could think of is that a student had failed to go full rich for takeoff, and that scared the school into implementing the "no lean for taxi" rule. (Now, as a member of a club, I lean aggressively for taxi.)

Instruction from my CFI went thusly: switch to rough mag, elevate RPMs a bit (2k?), lean it out until stumbling then enrichen slightly and run it like that for a bit. Switch to both and then do mag check again. :dunno:

So now I'm learning differently. I should be doing a similar procedure but on both mags. Apologies to the OP for adding my own questions, but I think they help answer his as well.
Click to expand...

They were probably worried people wouldn't go full rich for takeoff...at certain power / mixture settings at sea level you could easily have it set up to kill the motor on the takeoff roll or right after rotating...
 
Cpt_Kirk said:
I was taught:
Mags both
Full mixture
Full static rpm
Lean peak rpm
Wait 60 seconds
Full mix
Throttle roughly 1800
Check again

Repeat if necessary. If no improvement, return to parking.
Click to expand...

you don't want full mixture because the engine won't run as hot...you also can't do full static RPM in a lot of planes because the brakes won't hold it...seems a little dangerous actually
 
asechrest said:
Hmm. Both instructors I have time with indicated that I should leave it on the rough mag when trying to burn off the fouling. So I guess I'm in the same boat as the OP: been doing it wrong?

Ahhh, the joys of being a low-time pilot. :D
Click to expand...

Yep, doing it wrong.
 
Pretty sure both Continental and Lycoming recommend against the practice of burning carbon off a fouled plug.

Though admittedly in practice it does often work.
 
FastEddieB said:
Pretty sure both Continental and Lycoming recommend against the practice of burning carbon off a fouled plug.

Though admittedly in practice it does often work.
Click to expand...

It's not typically carbon, typically it's lead that fouls the plug.
 
Ahhh, the joys of being a low-time pilot. :D

And being stuck with low skill instructors
 
Always talk to your mechanic,he should know the engine better than a new instructor.
 
FWIW

Per C172 checklist. I'm suppose to lean the engine at 1000RPM
During the Mag check if the mixer is not lean, the motor runs rough.
Prior to take off mixer is full rich.
 
evapilotaz said:
FWIW

Per C172 checklist. I'm suppose to lean the engine at 1000RPM
During the Mag check if the mixer is not lean, the motor runs rough.
Prior to take off mixer is full rich.
Click to expand...

Which 172? The SPs say to do that at 1200 RPM. N's say leaning for ground operations is recommended.

Full rich is incorrect in a 172 above 3000 density altitude at any time perhaps excepting engine start (for carb'ed 172s). You're gonna see that a lot in Arizona in summer. Maybe that's why it's rough.
 
MAKG1 said:
Which 172? The SPs say to do that at 1200 RPM. N's say leaning for ground operations is recommended.

Full rich is incorrect in a 172 above 3000 density altitude at any time perhaps excepting engine start (for carb'ed 172s). You're gonna see that a lot in Arizona in summer. Maybe that's why it's rough.
Click to expand...

C172N 1200ft Elevation. After engine start 1000 rpm lean mixer. I have demonstrated engine roughness because I wasn't lean enough.
 
mrkbbd said:
you don't want full mixture because the engine won't run as hot...you also can't do full static RPM in a lot of planes because the brakes won't hold it...seems a little dangerous actually
Click to expand...

Aside from the issue of "FULL static rpm," isn't the checklist OK for burning off fouled plugs since after "full mixture" it specifically includes "lean peak rpm", as bolded here?:

Mags both
Full mixture
Full static rpm
Lean peak rpm
Wait 60 seconds
Full mix
Throttle roughly 1800
Check again
Click to expand...
 
You might burn off carbon fouling, doubtful you'll get rid of lead short of pulling the plug and cleaning it physically.
 
evapilotaz said:
C172N 1200ft Elevation. After engine start 1000 rpm lean mixer. I have demonstrated engine roughness because I wasn't lean enough.
Click to expand...

I didn't ask elevation. I asked density altitude.

Gilbert (Phoenix) could easily have DA above 3000 for half the year or more. For that, you take off leaned to best RPM after static run-up. Read your POH.

Use high altitude procedures in a 172(N) when the DA is above 3000. That means no full rich mixture, except for engine start (even then, only because the DA isn't THAT high).
 
Ok Makg

Anyway as I just got current after spending 11 hours dual the CFI never mentioned taking off Lean. He had me take off full rich. Time to re read the POH

Thanks.
 
I own my airplane so I'm not subject to stupid club rules or stupid club maintenance and only one stupid club pilot - me! I had been lax about leaning for taxi until I had a plug fail that couldn't be "burned clean." It was fouled with lead and had to be removed and cleaned. I mean I leaned for taxi because this is Denver and DA gets up over 9000' in the summer, but not nearly as aggressively as I do now. I lean for everything always now - often even for start. Lesson learned.

If you are flying the airplane you are the PIC and you are the one whose butt and passengers are in the airplane. Fly it the best and safest way you know how.

Personally, I wouldn't want to run the engine excessively rich and have cylinders dropping out. And as far as the club's justification for its rule goes, the problem was not the leaning for taxi but the improper use of checklists since the student would have failed to follow the before takeoff (runup) checklist. Then the club overreacted.
 
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