Roughness - Runup

Hello,

Our Aviation Air Explorer Scout program has a C-172 M 1974 model. I am a CFI with the group and today during run-up did the traditional mag check. During runup the left mag small drop nothing unusual. Though, when we do 2 clicks left and check the RIGHT MAG (Serious engine roughness) and large decrese in RPM. Any thoughts, that can help me to discuss with our A&P/IA during normal hrs. on Monday? Do you think it is a bad set of mags or just one? I know every 500hrs they are to be inspected and/or overhauled.

jetpilot@gmail.com

THANKS

It could just be fouling of the plugs. Do you lean aggresively on the ground? Did you try to "burn off" the deposits by adding full power on Both Mags and then leaning the mixture?

Michael

Yes! I did try Full Power and lean mixture to do exactly what you posted as a suggestion. That did not work out this time! The issue remained after full pwer and leaning of mixture. In addition to that the spark plus all 8 of them are new as of May/June 2006.

Thanks

Gotta be a bad magneto then.

Do you have individual cylinder CHT/EGT monitoring?

Responding to Berkut question.....Our A/C does not have an individual CHT/EGT monitoring system. However, our EGT is placard permanently INOP so we lean to engine roughness and then push in till smooth. Thats why I think it could be a mag issue.

sometimes the plugs get lead or oil fouled so bad the only way to fix it is to pull the plugs and clean em.

I've found that a lot of people, even after noticing a fouled plug, do not leave the mixture leaned for long enough. I've noticed that it may take as much as 5-10 minutes to burn off a severely fouled mag...

PILOT754.....tell your A&P/IA exactly what happened. What he/she will do before throwing a new magneto on it is pull the plugs corresponding to the right hand magneto (lower right/upper left plugs) and clean them and test them on a spark plug tester. 90% of the time, a plug was too fouled to burn off by leaning the mixture as STUNTGOAT eluded too. It'll take about 15 - 20 minutes to get the plane back up running again--no big deal and very cheap to fix indeed. Another technique that SIEGEL hit on was aggressive leaning on the ground...it's a good idea to lean aggressively for your taxiing out to the runway.

BTW, removing cleaning and replacing sparkplugs is listed as "preventative maintence" in Pt 43. Any private pilot owner can do this themselves, no A&P required.

I'd be careful about this. I'm definately not an A&P, but my old boss was and he was fanatic about not leaning and powering up for a long period of time unless the aircraft had EGT's/CHT's. His theory was that spark plugs can easily get so fouled that it's impossible to burn off the deposits, and he believed that leaning agressively would potentially damage the engine with high cylinder temps.

I don't know whether he was right or not, but for a flight school his engines had remarkably long times between overhauls.

It wouldn't be much hotter than doing the same thing in the air...

How long did you leave it leaned out for on the ground? I wouldn't use full power either... maybe 2000 RPM or so and then lean it; leave it there for a couple minutes and then try it.

Yup.

Again, I don't know how long you can run it up on the ground with the mixture leaned, but I sure wouldn't want to do it for long without a pretty good way to look at the CHT's.

Once I was with a student, and the runup went fine, but on the takoff roll there was some pretty bad engine roughness. We taxied back to the runup and did another mag check, and both maggs were running rough. I figured it probably wasn't a fouled plug since both of them were running rough, but it sure felt like one. We tried buring them off with no success. We taxied back to the hangar and they opened it up to see what was up. There was a bunch of oil leaking from the no 3 cylinder so they opened the valve cover and the rocker arms fell on the floor. The cylinder head was practically shattered. Glad the problem showed up on the takoff roll, and not shortly after.

Another time I took a student to a checkride. After they came back (he passed BTW) the examiner mentioned that the left mag ran very rough in flight, but before shutting down they burned it off and it seemed to be running clean. We all agreed it was probably just a fouled plug. We then flew back to our home airport. The first ten minutes of the flight the engine ran beautifully, but then the left mag started acting up again. I did a mag check in the air and when I switched to the left the engine began detonating, so we completed the flight on the right mag. After we landed and began taxiing to the hangar, the engine quit. (I'm glad my first engine failure happened on the ground) What's weird though, is we were able to re-start the engine and after the re-start both mags seemed to be running fine.

If you haven't started this practice yet, it's a good practice to leave the mag selector set where ever it got set prior to departing, until you land...when single engine. You never know what messing with that is gonna do. The last thing you need is to go from both to left or right and have the engine quit and not be able to restart. It should restart if you go back to what was working but you never know...

Leave the mag troubleshooting to on the ground or if you absolutely have to do it inflight (you really shouldn't have to) stay in the pattern. Most engine issues can be troubleshot with a ground run.

I have to disagree. The whole reason we have a mag selector is so we can isolate a bad one. If the timing slips on one (Which is what I think happened in my case), it can cause some serious engine trouble. Switch to the good one and it will at least get you to a nearby airport. When I swithced to both, the engine backfired! I wasn't about to let it run like that when it ran perfectly smooth on the right one. And the setting at takeoff should always be both. If either mag is not working prior to takeoff, abort the flight.

This is very true. I've personally used a spark plug cleaner and tester. The plugs when fouled are terrible to clean, and the cleaner is like a little jackhammer that literally hammers the hardened residue out of the plug. Not likely to burn that off.

The way I used to do it was 2300 rpm for a fixed pitch prop or 23" MAP, then lean for Peak EGT if there is an EGT gauge or mid way between peak RPM and it running rough. Do this for 30 seconds at a time and try another mag check.

If you have a CHT gauge, you will be suprised at how fast the engine heats up. I would only do this 2-3 times before you call it a day.

If you have a CHT gauge, you have more leeway, but be careful. Usually after 4 attempts at this, the engine was really getting hot and if you haven't burned off the fouling after 3 attempts, my experience is you are screwed anyway. The Redline is often too hot for comfort in most airplanes. You don't want to go any hotter than 350 for sure. It is not worth overheating an engine when a mechanic can easily fix the problem.

If your airplane is prone to fouling, you can minimize this by leaning on the ground as much as possible. You will be suprised how much you can lean the mixture (in many cases, almost to cutoff) on the ground. If you do this, lean as much as possible, it is safer this way. If you only lean a little bit, you are not helping the fouling issue and it is possible to takeoff with the mixture leaned, which is a very bad thing to do (it can cause permanent engine damage or engine failure). If you lean it as much as possible, you cannot takeoff this way, the engine will simply not throttle up.

Whatever you do, do not sit and try to burn off the plugs for 5-10 minutes at a time. Your mechanic will love you, but who ever owns the airplane will not be happy when they have to caugh up thousands for a new engine.

Rightously so (you didn't mention going back to both, you just said that you did a mag check)...and the reason there are L and R is because we need to isolate each mag on the ground. What I meant was, don't change what you have unless you have to. There's a reason most emergency checklists tell you to make sure you're on both, but they don't mention to do a mag check while inflight. I probably would've done the same thing if I had to, but it would've been one of the last things I did to try and fix the problem. If you know for sure that it's one specific magneto and the engine is running rough enough, then do the most logical thing. If I was considering going from both to the right mag inflight, it would have to be for a good reason, because that means crossing over the left mag, which in your case, was acting up. We both know that with recipricating engines, even if it decides to quit, you can still get it right back if you make the right moves (in cases similiar to yours), but why chance it unless you know, for sure, that your either gonna stop the engine from eating itself or get a nice improvement in performance. In your case, it seemed to help but I still think doing a mag check inflight was not the wisest thing to do. Again, do what you have to to prevent losing the engine completely (or get it back) or get a much needed boost in performance. Mag checks are for the ground.