Mag Drop

[Floppy Link]

Hi folks, pick yer collective brains if I may....

Picture the scene...PFA aircraft coming up for permit renewal.
Bit of a mag drop on groundrun...

RPM 1200 1400 1600 1800

Carb -30 -40 -40 -45
L Mag -80 -70 -80 -80
R Mag -90 -150 -160 -130

Cleaned the plugs - no change
changed the plugs - no change
checked ignition harness - seems correctly rigged iaw Continental O-200 manual

the next steps involve stripping hoses, baffles, exhausts etc to get at the mags at the back of the engine in order to...
1) remove and test the ignition harness, then
2) remove and bench test the mags

Long shot I know but can anybody think of anything else we can check before going down this route? It looks like it's going to be expensive so I'm clutching at straws here....

Russell :\ :mad:

[Arclite01]

Left mag knackered - or maybe just damp.

Try drying it out - was the aircraft left outside or in a hangar ?

[Flik Roll]

is it fuel injected? If it is...try leaning the mixture while on the dropping mag.
Often if you leave fuel injected a/c running on fully rich the mags get dirty etc.

[Zulu Alpha]

Try checking the mag drop after a flight when everything is warm and after you've been flying for a while at cruise setting with the mixture leaned.

If it is better than just after a start then try drying everything and/or changing the spark plugs.

If there is no difference then I would still try changing the spark plugs before paying lots of money to have the mags overhauled. A friend of mine had an engine that kept stopping when the throttle was closed. It all got better when he changed plugs.

[IO540]

Often if you leave fuel injected a/c running on fully rich the mags get dirty etc

What would cause this? I didn't know the fuel/air mixture got inside the magnetos.

[A and C]

Move the aircraft into a dark hanger and with the high voltage ignition tester do the check in the dark , what you are looking for is sparks escaping from the ignition harness.

The ignition tester may be giving a servisable reading but the leads may be only just breaking down.

Doing this has fixed the starting problems on at least two PA34's.

[shortstripper]

Ok .... Off on a complete tangent here, but a couple of things worth checking.

1. Are the intake tubes nice and tight? The rubbers each end can become hard and if the jubilee clips aren't that tight they can become loose. This can cause a lean mix on one pot, which can lead to plug or worse valve failure. Usually this will not cause a mag drop but shows as a slightly rough running and especially in the mid revs range. However, this can appear worse as one plug is switched off. Probably not the cause, but it's a simple check that's worth doing and is often overlooked.

2. Mag timing, remembering that the left mag is usually a couple of degrees retarded over the right, is it possible the timing on the right mag may have moved slightly too advanced? Are the nuts good and tight? Are the mags secured using spring washers, or stiffnuts and not nylocks ... the latter can theoretically loose their grip due to heat cycles and shouldn't be used on engine componants really. Being a PFA aircraft, never dismiss poor aircraft practices from previous owners.


Left mag knackered - or maybe just damp.

Right mag is the one with the bigger drop?


What would cause this? I didn't know the fuel/air mixture got inside the magnetos

I think Flik must have meant plugs?

All other advise re damp, knackered leads ect very valid. It could of course simply be a case of knackered mag though :E

SS

[Flik Roll]

IO540...
Im not an engineer or an engine boff; however I probs did use the wrong terminology but on the CAP10c; when doing the mag checks, if there is a large drop or drop difference we have to lean the mixture on that mag or at both then try again and it always solves the problem.

On t'other type I fly, we set the mixture as part of the after start checks (Setting best power mixture) so mag drop problems are rarely a problem. :ok:

[IO540]

"we have to lean the mixture on that mag "

OK, what you are doing is leaning the mixture on that set of spark plugs not on that mag

This is an attempt to clear debris on a spark plug(s), by running a higher EGT than would normally be the case on the ground.

What I am curious about is this: as I well know from my IO-540 experience, a bunged-up plug makes the engine (and the whole aircraft) vibrate like hell during the mag checks, and one doesn't need to look at the RPM to realise that one has a duff plug. (I can also see it on the EDM700 which shows a zero EGT at that time on that particular cylinder).

Is it possible to have a partially bunged-up plug, just enough to cause say an extra 50rpm mag drop?

[Flik Roll]

Don't ask me...I dont do engineering :\

[foxmoth]

Troubleshooting suggestion -
Run the engine on the bad mag until it is warm, shut down and carefully check the cylinder temps - the duff one should be considerably cooler than the others. Swap the plug from that side mag to a good cylinder (make sure you get the correct plug or swap them both to be sure) and repeat. If the cool cylinder has changed you know it is a plug/cylinder related problem, if not you know it is further back.
If you have a mag drop you cannot clear and taxi in on the bad mag it gives the engineers a good clue where to start due to the cool pot.

[Floppy Link]

Thanks for all the replies folks...food for thought
:ok:

[Mark 1]

One quick check.

Swap plugs from top to bottom. Does the mag drop swap sides?
If so it's the plugs; if not, then probably mags or leads.