Mag Checks - A cure for rough running.
 

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.

Thoughts?

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.

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.

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.

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:

brgds
BB

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.>>

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"

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.

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.

John

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.

Mydata

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.

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.;)

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.

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.

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.

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.

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.