Mag Checks - A cure for rough running.

[White Bear]

Foxmouth,
“and you do NOT run at high RPM on the one mag”.
Why not?
Isn’t that exactly why it’s there?

Far too many old wives tails on this thread.
Might I suggest John Deakin’s excellent articles on engine mixture management on avweb. (http://www.avweb.com/news/columns/182084-1.html)
They will be most educational for some, and a revelation for others.

IO540’s & 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.
Regards,
W.B.

[Leezyjet]

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).

[foxmoth]

“and you do NOT run at high RPM on the one mag”.
Why not?
Isn’t that exactly why it’s 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.

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

[IO540]

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 But having loss of ignition on one cylinder seems more probable.

[Say again s l o w l y]

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.

[Tarq57]

White Bear,
That article was both educational and revealing. And an easy and entertaining read. Definitely added that site to the faves folder!

[FlyingForFun]

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?

FFF
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[Andy_RR]

Originally Posted by FlyingForFun

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?
FFF
---------------

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]

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.

[IO540]

FFF

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?

[FlyingForFun]

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 EGT

Thanks, 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!

FFF
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[Say again s l o w l y]

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!

[Tarq57]

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.

[englishal]

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]

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.

[LowNSlow]

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]

Prevention is better than cure !

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.

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

[IO540]

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]

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.
Regards,
W.B.

[A and C]

IO540

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.