Mag question/200rpm drop

[Tom G]

HI everyone:

February here in New Jersey was awful. My plane wound up sitting since the end of January and I just flew it for the first time today. At runup the left mag was rough and had about a 150-200 rpm drop. I figured the bottom plugs may have fowled and that a short flight in the local area would clear things up. Well, I flew around for over an hour and the situation really didn't improve. The points were changed 21 hours ago and it was running fine the last time I flew it. We checked the bottom plug wires and all looks fine. I didn't pull the plugs since I didn't have the tools with me today but I'm planning on doing it next week. Whatever is wrong went wrong while it was sitting. Its definitely not an internal engine problem because it runs great on the other mag. Any ideas??

Thanks!

[whake]

Re: Mag question/200rpm drop

The first thing to do is clean the plugs and check the gap, a little corrosion on the gap can lead to early arcing on just one plug. Look for lead deposits deep in the plug, carefully clean.

[Pearson]

Re: Mag question/200rpm drop

Tom,

Start the engine, then switch to the mag with the RPM drop. Let it run for 5-6 minutes ON THE BAD MAG. Turn off the engine and check exaust pipes near cylinders to see if one is noticably cooler than the others. If you have one of those laser light temp sensors, now would be a good time to try it out.

If one of the cylinders is a LOT cooler (COLD) than the rest, you have a bad plug, valve, wire, etc.

If they are all about the same, the problem is probably in that mag.

Please let us know what you find.

[cpirrmann]

Re: Mag question/200rpm drop

You can use a crayon to see which one it melts on and which it doesn't

Tom,

Start the engine, then switch to the mag with the RPM drop. Let it run for 5-6 minutes ON THE BAD MAG. Turn off the engine and check exaust pipes near cylinders to see if one is noticably cooler than the others. If you have one of those laser light temp sensors, now would be a good time to try it out.

If one of the cylinders is a LOT cooler (COLD) than the rest, you have a bad plug, valve, wire, etc.

If they are all about the same, the problem is probably in that mag.

Please let us know what you find.

[Tom G]

Re: Mag question/200rpm drop

Thanks Guys, great ideas!.............I'll try this next weekend and let you know

[pftcraft]

Re: Mag question/200rpm drop

Would I fly if I knew one set of plugs or mag was not working...? Call me chicken but I think I'd repair the problem first...

[fearofpavement]

Re: Mag question/200rpm drop

A 200 rpm drop on one mag has always been a non firing plug in my experience. Not that it couldn't be something else but if it runs fine on one mag that eliminates mechanical issues such as stuck valves, mouse nest in the intake tube or other weird stuff that can happen to a sitting plane. (ok, forget the mouse nest thing) Anyway, Once you clean the plugs I think your problem will be gone.

Regarding flying the plane with one plug not firing I don't feel that would be unreasonable for a local pattern flight.

The ideas posted for finding a cold cylinder are effective and also a good diagnostic tool to go directly to the problem cylinder without having to do all the plugs. Admittedly, on a T-craft pulling all the bottom plugs is in a different category than a plug removal on a two row radial engine, but time is time...

[Howard Wilson]

Re: Mag question/200rpm drop

Yeah sounds like a plug. The plugs on the open cowl Lycoming are easier to check being outside the cowl and both plugs on top, but the process of eliminating the good plugs (could hurt) and finding the bad could be done with the cowling off. Installed a set of new plugs on 20442 this year and about every hour one would fail! no obvious faults, internal shorting??? Those failures would give a 150-200 rpm drop. To find the culprit I would have the TAIL TIED DOWN, warm it up and let it idle, always holding on to the front strut with one hand put my other hand behind the exhaust outlet and feel the exhaust pulses, looking for a ragged pattern. On the Lyc. with dual exhausts this would narrow it down to two cylinders. then knowing which mag is low follow each of that mags wires out to it's plug. With the engine still at low idle take a long insulated handle screwdriver and short each suspected plug to ground, cylinder, exhaust pipe, whatever's close. When you hit a good plug the engine will falter noticably. When you ground the bad plug the engine rythym will not change. You found it! On the Continentals with the cross over exhaust you'll have to follow four wires out to their plugs from the low mag.

[1938BF50]

Re: Mag question/200rpm drop

Howard,

So why were you failing plugs every hour.....I am curious if you found the reason?

[Howard Wilson]

Re: Mag question/200rpm drop

Mike,
The old mechs I talked with said they were probably dropped somewhere along the line. Had three out of eight fail. The one mech I talked with said he was taught "you could only drop a plug twice; once on the ground and once in the trash!"

[Hank Jarrett]

Re: Mag question/200rpm drop

My IAs father (also an IA) told me once that you could always make one more flight with a dropped plug, at which point he would throw it as hard as he could out into the field next to the airport. MAN! That old fart had an arm!!! You couldn't even see where the plug landed.
Hank

[Howard Wilson]

Re: Mag question/200rpm drop

My IAs father (also an IA) told me once that you could always make one more flight with a dropped plug, at which point he would throw it as hard as he could out into the field next to the airport. MAN! That old fart had an arm!!! You couldn't even see where the plug landed.
Hank

Ha Ha, I like that line better.

[Tom G]

Re: Mag question/200rpm drop / Problem Fixed!

I want to thank all of you for your advice on this one...Chris, the crayon idea worked great.

I started it up and switched to the left mag where the problem was. Let it run for about 5 minutes and the crayon melted on all the stacks except for the left front cylinder. I pulled the left bottom plug and switched it with the top one and believe it or not the left mag still checked bad. The plug had looked fine but I cleaned it up anyway. We had a wire tester and tested the wire from the mag to that plug and it checked good. We then discovered though that the wire coming out of that mag was probably a half inch up from the bottom of the cap. The mag had the points changed back at the annual 22 hours ago and I can only guess that the wire was never put all the way back into the mag and it must have been arcing all that time and it finally just decided to stop.

Anyway, here is the part that really freaked us out. We left the bottom cowl off and used some bungee cords to make sure the top cowl wouldn't flap and the nose piece wouldnt hit the prop while the engine was running. We started it up and the left mag was now fine but the Right one was rough!!! It turned out that the nose cowl due to the bungees pulling it back was grounding out the left top plug! Anyway, once we discovered that problem we put everything back together and it ran perfect.

Thanks again for the advice since that enabled us to find the dead cylinder without pulling all the plugs.

Make sure those wires are all properly inserted into your magnetos!!!