Larger than usual mag drop....

[topcat450]

Yesterday, started up the aircraft no probs, spent a while getting myself all sorted and letting the engine warm up, then a longish taxi to the hold point (all at low power). When it came to doing the power checks I was getting a large drop off on the left mag only. I tried it several times, each time with exactly the same effect.

I got out of everyone elses way and increased power to the max I could with the brakes holding for 30 secs or so and then did the mag checks again - everything had returned to normal. I then reduced power, mag checks were fine... I tried it at several power settings, carb heat in, out.. every combination I could think of and it was all back to normal.

Was this 'plug fouling' that I seem to recall reading about somewhere, or does it sound like something else?

[Circuit Basher]

Seems a pretty classic example to me. Common after long taxis, where surplus oil or rich mixture combine to cause deposits on one or more of the plugs. Sounds like your actions cleared it - I'd generally lean the mixture as well to increase the engine temp whilst trying to clear the problem, but then return to max power mixture prior to power checks (otherwise you might overheat the engine).

[A and C]

If you suspect plug fowling then running at max RPM is OK but don't lean off as the rapid temp rise is not good for the cylinder heads.

You might also like to try the Lycoming recomended shut down- run the engine at 1600-1800 RPM for 30-45 sec then set 1200 RPM and shut the mixture.

This is stated in a Lycoming SI to cut down on plug fowling.

[FullyFlapped]

I used to frequently encounter plug fouling until I started the "Lycoming recommended shut down" (per A&C above) - no problems since.

Also recommend you taxi with a leaner mixture (obviously as long as it's OK with your POH), restoring full rich before power checks.

FF :ok:

[jayteeto]

The answer to your original question is: Yes, that is plug fouling and what you did was the correct first actions, which worked. If that had not wrked you need to be familiar with the recommended actions for your engine. Usually there is an RPM to set and the mixture is leaned to an approved setting. This normally works, otherwise you may have a problem with the mags. Remember, this is the very reason you do the check so well done on being thorough!!

[topcat450]

Thanks for the advice folks - 'tis slightly un-nerving when it first happens, I guess I'll be quicker to recognise it in future. As I don't have a mixture control I guess not hanging about at really low power settings is the simplest preventative action.

Thanks again.

[IO540]

Presumably, if one gets substantially uneven mag drops, and one has a multicylinder monitor (e.g. an EDM700) showing that all the plugs are working, it has to be a magneto problem?

What is the wisdom of going to fly in such a situation?

[Sensible]

Depends really on whether you have done the approved crash course or not ;) :cool:

[foxmoth]

As I don't have a mixture control

But you do have carb heat, Just curious - what aircraft?

[2Donkeys]

IO540

If you have a device such as an EDM, and magneto problems, the evidence will be plain to see in the form of either dead cylinder(s) or lumpy EGT peaks. This is all the more clear if you also have GAMIs.

2D