View Full Version : Mag Checks - A cure for rough running.

5th Feb 2006, 15:03
During my PPL training I recall a time when during the power checks in a PA28 the engine began to run roughly on one mag. The instructor took over and did some high revving and messing with the mixture for a minute or so and then the rough running went away. It was at that time during my training that I already had too much to remember so I forgot the details of what happened.

I recalled this yesterday when doing power checks on a PA28, there wasn't any rough running but I wanted to recall what was done to cure any rough running if it did appear during the power checks. I'm no longer in touch with the instructor so can't ask what it was that he did.


5th Feb 2006, 15:17
It is possible that some plug fouling may occur with certain PA28 high time engines following protracted taxying. There is a well know cure for 'burning off' this fouling, but it must be very carefully followed to avoid engine damage.

Basically, it consists of setting full power, then leaning the mixture for a short period, before setting it fully rich again and then throttling back to 2000 rpm to repeat the magneto check.


5th Feb 2006, 16:16
Better to prevent than to cure: lean the mixture during taxi. Lean it until the RPMs rise and then drop. Keep it there until you do the runup. Last check before entering the runway: set mixture full rich.

If you would forget to set it full rich and you would apply full power, then it should make it clear to you, otherwise you didn't lean enough.

The only times I have fouled plugs are when someone else flew my aircraft previously.

5th Feb 2006, 16:20
Unlikely that the fouling is caused by mixture alone - more likely to be oil in the pots following protracted taxying.

Unless at high elevations, I would advise against lean taxying. Better to waste a bit of fuel than to burn the valves and pots!

5th Feb 2006, 16:25
i was told that it was impossible to damage the engine while doing a lean taxi. If you put enough demand on the engine to damage while on a lean taxi it will just stop.

Certainly what my engine overhaul company told me.

5th Feb 2006, 16:40
To do any harm with the mixture control you have to be at 65% power or higher. Very unlikely during taxi unless you didn't remove the tiedown ropes.

5th Feb 2006, 16:50
Just come back from my lesson and had a similar problem, however it was on a cessna 152 so i'm not sure whether you could remidy the problem the same way, but here it goes.

Increase the throttle to approx 2000rpm and then lean the mixture until you get start to get a slight drop in the rpm. hold this for maybe a 1 min and then set mixture fully rich. set power back to its original setting and try the magnitos again. As i said it worked for me but it was on a cessna 152. Another method my instructor showed me on a previous occasion was to leave the power as it is and lean the mixture until you get a drop in rpm and leave it like that. This also worked in my case.:ok:


5th Feb 2006, 17:07
What I meant was that the person who leaned for taxying might forget to reset the mixture for take-off and thus damage the engine by taking off with the mixture excessively lean at full throttle....

BB - I would be wary of describing the technique shown to you for the information of others. 1 minute at weak mixture and 2000 rpm is actually quite taxing for the engine - if someone does that and cooks the engine they won't be best pleased!

5th Feb 2006, 17:48
Well........ according to my engineer if you have it leaned for taxi and then apply full power still leaned the engine will stop.

I will test this in the morning and report back....:cool:

<<edit: Been there, done it - yeah, it stopped (well, it would have if I hadn't caught it quick enough). So much for "Cleared for immediate take-off". Bugger.>>

5th Feb 2006, 20:12
What I meant was that the person who leaned for taxying might forget to reset the mixture for take-off

I thought that was why pre T.O. checks were done after the engine run - and part of the pre T.O. checks should be "Mixture Rich"

6th Feb 2006, 00:15
Air cooled aero engines (ie most) default to over rich with the mixture forward, as it provides extra cooling from unburnt fuel during takeoff and initial climb, where cooling airflow is low.It does this by design, and is necessary. This over rich state is a great deal richer than best power mixture. So you can take off with a lean, or leaner than T/O power mixture,(if you did this and forgot the t/o checks) and the engine will run just fine, until it cracks a head or burns out a valve or any of the other nasties this may cause.The engine would need to be significantly overleaned for it to die when the throttle was opened.I always use full rich for any ground operation (unless hot'n high, which you should have training for).Leaning a bit on the ground will produce a slightly higher combustion temperature, which helps prevent crap fouling the plugs,plus there is less unburnt crap,but I think it would take a long time to overheat an engine this way.
We had a lot of problems in the 80's when the fuel grade changed to 100LL, and found plug fouling after as little as 3min of ground running quite common on some, generally older engines.If this occured, the technique was to lean for maybe 10sec at runup power, to about best power mixture,then try the mags again at full rich.Sometimes needed repeating for a little longer, or at slightly higher power. If it made no difference after the first attempt, there was something wrong with a lead or a plug.

6th Feb 2006, 01:02
I'm pretty surprised by the people who don't lean on the ground. This seems to be standard practice in the US, iirc all of the student pilot textbooks recommend it. I can't imagine why anyone wouldn't do it. I can tell you that on my plane (TR182, Lycoming O540) running rich on the ground for any length of time guarantees fouling of the lower plugs.

If you do it properly there is no risk of taking off with the mixture too lean and damaging the engine. Just lean until the engine speeds up a bit, then richen by a twist. Applying full throttle or even enough for a fast taxi will stop the engine very quickly. (Of course if you have a GEM you can watch the actual temps anyway, and if you don't have a GEM, well why not? You are just asking to mismanage the engine one way or another).

If you DO get fouled plugs (rough running on a single mag) the remedy is simple - especially if you have a GEM. My procedure: run up to full take off power, lean and manage the mixture to keep TIT at around 1450 until the CHTs are up to normal level (about a minute). Of course without a GEM it's harder to be sure and it's prudent to limit to 65% power - then you can just lean until you get rough running and back off a bit, again without having to worry about damage - though watch the CHT if you have one.


6th Feb 2006, 05:40
Thanks for all the very useful responses. Very informative. I'll have further discussions with the CFI next time I'm at the school.

Something that got me thinking further on this (and I guess the answer is in my text books). If the engine runs roughly on only one mag why isn't this the case when both are selected - it is as though the rough running can only be detected when mags are selected independently. And if a single mag is running roughly, and on both things are fine, what amount of total power is available at take off? I would expect less than normally expected due to one of the mag circuits being less than perfect.

6th Feb 2006, 06:59

What the instructor did was to clear a bunged-up spark plug. What you do is you set the mag switch to BOTH, rev the engine up to quite a high power setting (say 2000rpm) and lean it back very aggressively. This makes the combustion a lot hotter than would be the case with mixture full-rich and it "should" clear the plug. I've had cases where it didn't and had to take the plug out. Then... which plug? A multicylinder engine monitor makes it easy. Nearly always it is one of the bottom ones.

Leaning on the ground should always be done. The engine cannot be damaged at such low power settings. Exception: taxiing on rough/wet grass; this can require a lot of power especially if trying to initially move the plane after a few days' parking in a soft spot. Then one has to enrichen the mixture.

6th Feb 2006, 08:20
. Then... which plug?

One way to save a lot of searching is to taxi in with the rough mag selected, then when you shut down the duff cylinder will be a lot cooler than the others - now just a choice of 2 instead of 8 or more.;)

6th Feb 2006, 08:32
Never thought of that one :ok:

However, the engine will be shaking itself to pieces during that time. Even my 6-cyl one is very rough, even on idle, if there is a cylinder not producing any power.

Say again s l o w l y
6th Feb 2006, 09:14
Oh god, don't actually try that please anyone!!:eek:

With bigger engine (360 and bigger) then leaning whilst taxiing is definately recommended. (It's certainly in the POH most of the time.) But with little engines (O200 etc.) then they shouldn't foul up as readily. At least not at low Density Alt's.

If you have a prolonged period at low power, then fouling is a distinct possibility on any engine.

To clear it, follow the advice already given, increase RPM and lean the engine for a short period. If this doesn't clear it, try again, but be aware that mag problems are sometimes caused by other issues that require an engineer to clear them.
For example I saw an FI sitting at a hold point the other day with an engine back firing, popping and generally sounding like it was about to collapse as he s*dded about trying to clear it, when there was really no doubt that it wasn't a case of fouling but something far more serious. The look of pain on the engineer's faces was something to behold.
So if it's really bad and it won't clear, give up before you do a lot more serious damage.

6th Feb 2006, 13:12
the engine will be shaking itself to pieces during that time.

If it is that rough it is likely to be a lot more than plugs! - and you do NOT run at high RPM on the one mag.

6th Feb 2006, 13:35
It's an interesting question whether it is an engine certification requirement that it must stay in one piece with one cylinder not developing power.

Say again s l o w l y
6th Feb 2006, 14:27
Whilst I have no knowledge of the certification process, I can't imagine that running on one cylinder less would be part of it.
There are so many different ways a cylinder could fail, from valve problems to the ever favourite cylinder coming off and leaving large lumps of metal flailing away. It's pretty difficult to replicate failures as there can be so many variables. It easier to to say it either works or not!

One dodgy mag or plug, shouldn't lead to there being one cylinder not working, rather one working slightly less efficiently, afterall, that is why we have a dual ignition system. Even if it is a decrepit, old school solution.

White Bear
6th Feb 2006, 23:24
ďand you do NOT run at high RPM on the one magĒ.
Why not?
Isnít that exactly why itís there?

Far too many old wives tails on this thread.
Might I suggest John Deakinís excellent articles on engine mixture management on avweb. (
They will be most educational for some, and a revelation for others.

IO540ís & N5296s comments on the use of a multi cylinder engine monitors especially when the engine is fitted with a constant speed prop, cannot be overstated.

7th Feb 2006, 01:21
I had plug fowling happen to me on my first solo after a prolonged hold on the ground.

I half panicked and considered taxiing back in, then composed myself and remembered that it had happend before so I sorted it out and off I went.

However, I ran the engine upto full power for abour 1 min, but did not touch the mixture (which was on full rich) then went through the checks again and it was fine.

Was that the correct thing to do ?. (This was in a C152).


7th Feb 2006, 06:58
ďand you do NOT run at high RPM on the one magĒ.
Why not?
Isnít that exactly why itís there?

WB - I am talking here about on the ground, taxiing in after finding a rough mag and people have said about high vibration. Even in the air I would try and keep RPM as low as I reasonably could if I had an engine rough running - and I can think of few situations where I would deliberately run it on the one dodgy mag if I was in the air.:ooh:

Leezyjet - no problem with your action, leaning the mixture makes it more likely to clear, but if it clears without it that is fine.

7th Feb 2006, 09:34
Some wires crossed here.

Nothing wrong with running at any RPM on just one mag. There should be little or no extra vibration when just one plug per cylinder is igniting.

What appears dodgy, and raised my question about certification, is running on one mag at high RPM when it is known (and very obvious from the massive vibration) that there is a duff plug, and thus one cylinder will not be igniting at all. This may generate additional stress in the crankshaft, although the picture of crank stress is mostly dynamic (inertial) so one pot not making power may actually not matter at all. It will however matter hugely to the engine mounts; I am sure that if I revved my IO540 at 2500 with one duff pot, it would eventually drop off the aircraft.

This is why clearing a bunged-up plug is done with the mag switch set to BOTH - the lead deposit is most unlikely to ever clear if there is no combustion in the cylinder.

Having said all this, I've had two cases in 3 years of plugs not clearing and having to come out. So now I carry the tools. These were fine wire iridium plugs, very expensive and should not do this. And I do lean on the ground, and every other time except during climb.

Obviously I would not expect and engine to be expected to continue running with a con-rod sticking out through the crankcase, and with the oil all over the windscreen :O But having loss of ignition on one cylinder seems more probable.

Say again s l o w l y
7th Feb 2006, 09:39
Running an engine on one magneto should do no damage at ANY time. A car doesn't have 2 spark plugs per cylinder does it. (apart from certain Alfa's that is.) The affected cylinder(s) won't be as efficient, but there should be no damage done. If there was, what would the point be of having a dual system?

I would be more worried about damage to the a/c in otherways if you just went to full power sitting still on the ground for a while to clear a plug.

Firstly, potential prop damage by sucking in stones etc.
Secondly, brake wear.
Thirdly, potential slipstream damage to anyone behind.(Especially since most people don't seem to think about this....)

To "clear" a fouled plug we need to increase the temperatures inside the cylinder to "burn" away the contamination, this can be done either by using a high power setting (with all the associated problems mentioned) or by using a lower setting and leaning the mixture to produce a leaner, hotter burn.

So if you need to clear a plug, use a slightly higher setting than the one you use for checking the mags and lean, rather than just going to a high power setting.

The primary balance of an engine is more to do with the large lumps of metal moving backwards and forewards in time with eachother, but there will be problems as the firing sequence will be out if one cylinder isn't working. Short term it probably wouldn't be too catastophic, but prolonged use will most likely munch it. An engine is a finely balanced thing so anything that knocks it off kilter will damage it somehow.

7th Feb 2006, 10:14
White Bear,
That article was both educational and revealing. And an easy and entertaining read. Definitely added that site to the faves folder!

7th Feb 2006, 11:11
A very interesting thread so far, but starting to get a bit confusing. So can I please summarise what I understand so far - and if I've got this wrong, someone please correct me?

- Rough-running when checking mags is caused by a fouled spark plug.
- This can often be cleared by increasing the exhaust gas temperature
- The best way of doing this is with mags on both, more power, and a lean mixture

- Running on one mag will not harm your engine if all the spark plugs are firing
- Running on both mags with one duff spark plug will not harm your engine
- Running on one mag, with a duff spark plug on that mag, will cause rough-running - and, at high power, this may damage your engine

I think that's summed up the majority of the replies on the thread, but have I got it all correct?

And finally, a question. My car only has one spark plug per cylinder. It frequently sits at idle power for extended periods. But I don't recall ever having a fouled spark plug on my car. Why is that?


7th Feb 2006, 11:23
And finally, a question. My car only has one spark plug per cylinder. It frequently sits at idle power for extended periods. But I don't recall ever having a fouled spark plug on my car. Why is that?

Two reasons:

- Automotive engines have thermostatically controlled cooling systems, so the cylinder head remains within a closer temperature range - or outside of the head/plug temperature range that promotes carbon formation and fouling.

- Because your car has electronically controlled air-fuel ratio (mixture) so that it is not continually running way richer than it needs (a la Lycoming etc.) If it were to be running at 9:1 or so, like some aero engines do when full rich, it may be just as susceptible to fouling.

When I get plug fouling, I was taught the following (on a C152):

- run up to the green zone (>2000rpm) for a few seconds (5 or so), then recheck mags
- run up to the green zone (~2000rpm), enlean slowly-ish until it coughs, then full rich and re-check mags
- brakes firmly on, run to full power, full rich for 5 or so seconds, then re-check mags
- return to base if still outside mag drop limit

Say again s l o w l y
7th Feb 2006, 11:36
Why the difference FFF? Because car engines have been designed properly!! Plus they benefit from 30 years extra development compared to your average Avgas powered dinosaur.

For instance my Spitfire doesn't like it when it's left idling and takes a bit to "clear its throat" when you first open up the throttle, why? Because the carbs need adjusting/rebuilding as the lumpy vibrations tend to make it very rich at low RPM's. Fun when it gets going though!

What we are trying to achieve by increasing the temperature is increasing the heating inside the cylinder, this will lead to a hotter CHT eventually, but that's not what we are trying to heat up.

7th Feb 2006, 12:02

The objective is a high EGT, to burn off whatever is shorting out the plug. Usually this will be a piece of lead.

Actually I wonder to what extent it is necessary to do this at the usual 2000+ rpm. One can achieve the same EGT at 1500 or less, just by leaning. Obviously the BMEP will be lower at lower powers; perhaps it isn't just the EGT that clears the deposit.

If you have a look at the complete set of plugs from an engine, the top ones will usually be clean (unless the engine is being run way too rich - probably the case with many flying school planes) while the bottom ones will have a generous build-up of hard lead deposits at the base of the centre electrode insulator. These deposits have to be (very carefully, to not damage the insulator) scraped out. Occassionally a piece of lead gets stuck across the plug gap.

Another good question is whether one should fly away with a duff plug. I suppose it depends on where you are stuck ;) I've had different answers to that one, all from very experienced pilots. Apparently the short always clears in cruise, within seconds.

Aren't you an instructor?

7th Feb 2006, 12:37
Andy and SAS,

The engine in my 1977 Triumph Spitfire, which is based on 1940s technology, certainly does not have a computer-controlled mixture! But it is water-cooled, and this is thermostatically controlled. Because of this, it doesn't need the over-rich mixture, which probably answers my question - thanks for your help. (Mine doesn't 'take a bit to "clear its throat" when you first open up the throttle', but they were built with so many different combinations of inlet design and carburetion, it's quite possible yours is different to mine, SAS.)

IO540,The objective is a high EGTThanks, that makes sense. I've updated my earlier post to reflect this answer.Aren't you an instructor?Yes, but that certainly doesn't mean I know everything, and I'm always looking to learn more. Thanks for helping!


Say again s l o w l y
7th Feb 2006, 13:12
FFF, my spitty is a bit different to standard! In fact about theonly original bit is the steering wheel boss, everything else has been changed in someway.

Apologies for the digression and back to the thread!

8th Feb 2006, 00:57
Another analogy, whilst reminiscing about dinosaur technology, is the good old open fireplace. Notice how sooty it gets in the first half hour of operation, or when another shovel of coal is put on? (over rich) Then as it burns down towards embers the flames take on the blue appearance, and the soot buildups glow and burn themselves off? (lean)
Perhaps all that is required to remove soot on the plugs is a lean enough mixture at or above the temp. required to do this. I've found leaned at about 1500 adequate. You can have the EGT high as you like but if there aint enough oxygen in the mix to allow the soot to burn, its going to stay there, methinks.

Flying For Fun,

[QUOTE]Rough running when checking mags is caused by a fouled plug.

Usually, but it could also be a defective plug, HT lead, or magneto. If burning it off using the method/s described in this thread doesn't work, then it probably is.

8th Feb 2006, 03:47
I always lean on the gound in hot climates or else IMHE you gaurantee to foul the plugs.....

Spent many a happy hour trying to set up the twin SU's on a 1300cc spitty....never quite managed it....;)

Say again s l o w l y
8th Feb 2006, 22:35
SU's went into the bin a LONG time ago, twin Webber DCOE 40's. Wonderful, but don't try and take them apart. Otherwise all you'll hear is Ping........ Scatter.......Phone call "Can I have 2 more please..."

Back to the original thread.
The article pointed out by White Bear was very interesting, but whilst giving good advice to those with fancy-pants monitoring it was a bit short for those with bog standard lumps.
Debunks alot of myths though.

9th Feb 2006, 08:19
My wunderplane has single lever operation, no mixture, no idle cut off and no carb heat. This is all done by levers not electronics as it was built in 1946. I do occasionally suffer from carb ice which is cured by opening the throttle a bit to increase the hot airflow. I do however suffer from fouled plugs fairly regularly for the following reasons:

a) It's in inverted four cylinder so oil collects on the plugs when left standing as evidenced by the cloud of blue oil smoke when starting up after a prolonged idle period.
b) It's an old engine so the piston, ring and bore clearances aren't "as new" which gives a bit of oil blow-by when running cold.

As I have no mixture control I can only run the engine faster than idle to clear the plugs once the engine itself is warm enough to take it. I usually run at around 1,800 rpm (with chocks in and the tail pointing clear of obstructions) with both mags on for a minute or two. If a mag check at 1,200 rpm shows they are still fouled then I run it at take-off power (2,000 rpm) for a minute. If it's still fouled then I switch onto the mag that has the drop (usually the left side with the massive electrode plugs rather than the right side with the fine wire electrodes), hop out and feel which cylinder is blowing cold air out. This is easy as each cylinder has it's own exhaust pipe. Then it's time to get the spanners out and clean the plug.

A and C
9th Feb 2006, 12:49
Lycoming publish a sevice instruction on this subject and they say that the problem can mostly be cured by the correct engine shutdown drill as follows.

Set 16-1800 RPM for 20-30 seconds
Reduce the RPM to 1200
Mixture to ICO
I will look up the Lycoming SI number so you can all read what the engine makers say about this problem.

9th Feb 2006, 13:27
I think, and this is purely a guess based on a few hundred hours behind an IO540, that a lot of plug leading that is discovered upon startup mag checks actually took place before or during the previous shutdown.

I set mixture to maximum lean (as lean as one can and still have taxi power) during the post-landing cleanup checks (which are done seconds after leaving the runway) and shut down by setting 1200 and mixture to ICO.

Just before shutdown I check all the plugs (do a mag check) on the EDM700, to make sure that they are all OK at that point.

Not sure why setting 1600-1800 would do something particularly useful.

White Bear
9th Feb 2006, 14:50
I follow essentially the same procedure as IO540, with my O360, and have never had any problems on start up. I was always trained to lean very aggressively during all ground operations except during run up.
The only thing I might add, and this only a preventative measure; when the mixture is pulled to ICO, and as the engine dies, close the throttle completely. It will reduce the shuddering as the prop stops, and will at the very least help prolong the life of your engine mounts.
A bit off topic I know.

A and C
9th Feb 2006, 17:22
The shut down RPM recomendations are from a Lycoming SI and I think that the reason is that the chemical that helps the TEL in the fuel vaporise will not work at low engine temp.

Leaning the engine during taxi will no doubt reduce plug fowling as less fuel is going into the engine and at a higher temp but I simply don't know if the increased temp of a leaned engine at taxi RPM is enough to let the chemical do its job and vaporise the TEL that is the reason for plug fowling in a well maintained engine.

I only throw this imformation into the ring as most of GA seems not to have noticed the Lycoming SI on the subject interestingly the RAF use this shutdown drill on the Grob Tutor.