Mag Question

[Tom G]

My right mag has been giving problems lately. On the ground it runs up well but in flight under load it runs badly. I just put the ercoupe cups on the top cylinders so yesterday we thought that possibly the caps may have been grounding the plugs so we removed them. I took off and on ititial climb the righ mag was fine. I figured we found the problem. Another check a few minutes later and it was rough again. In a climb on full throttle I couldn't get more than 1800rpm and it was running very rough. Switch back to the left mag or both and it runs fine. I thought maybe a plug or wire all of which is brand new but now I'm leaning towards a bad coil or condenser in the mag. I have Bendix mags. After landing it ran up well again? Any one ever have this problem. Am I on the right track?

[Mark Julicher]

Re: Mag Question

Tom,
It could be the magneto, a wire, or even a spark plug.

A little physics: and I beg your pardon if this is already trivial information to you. Air is an insulator. More air pressure (e.g., compressed in a cylinder) means a stronger dielectric and means a higher voltage is required to force a spark to jump across your spark plug. Low pressure (e.g., inside a vacuum tube or electrical systems at high altitude means low dielectric and sparks jump easily, but often to places you don't want sparks to go.

That said, at full throttle you are putting the greatest stress on your ignition system. If the spark can jump to an easier place than across the spark plug it will do so. For that reason, spark plugs are tested under pressure. If you accidently dropped a plug and cracked the insulator you may have spark jumping to ground outside your combustion chamber, and perhaps only during the highest cylinder pressure. A failed plug would be the cheapest thing to replace. Find someone with a proper spark plug tester and check out your plugs. Don't forget to use only a 6 point socket on the plugs, that is just cheap insurance against stress cracking the plug.

If it is not a plug, you might try swapping the wiring harnesses and see if the problem follows the harness.

If none of that works, it very well might be the magneto, although sometimes strange things happen because of dirty ignition switches or poor P leads. What you describe, however, sounds like it is occurring on the high voltage circuit, and several things in a Bendix magneto can be the culprit -- distributor, contacts, etc. Perhaps you can borrow a magneto and see if that corrects the problem.

[dewhitaker]

Re: Mag Question

Tom, just a tip for when you put the ercoupe cups back on. When I installed mine I glued a piece of insulating rubber on the cap and safety wired the cap so it could not inadvertantly screw down on to the top of the spark plug. I have 90 hours now on my Slick mag installation and love them.

Your problem sounds similiar to what I was experiencing with the old Eisemann mags. With both mags on, did not notice anything. But individually during full static run up, one of them would break up and lose RPM. At around 1700 RPM it would run fine. Never did figure out what was happening.

[Guest]

Re: Mag Question

My right mag has been giving problems lately. On the ground it runs up well but in flight under load it runs badly. I just put the ercoupe cups on the top cylinders so yesterday we thought that possibly the caps may have been grounding the plugs so we removed them. I took off and on ititial climb the righ mag was fine. I figured we found the problem. Another check a few minutes later and it was rough again. In a climb on full throttle I couldn't get more than 1800rpm and it was running very rough. Switch back to the left mag or both and it runs fine. I thought maybe a plug or wire all of which is brand new but now I'm leaning towards a bad coil or condenser in the mag. I have Bendix mags. After landing it ran up well again? Any one ever have this problem. Am I on the right track?

Hi Tom,

Some random thoughts,

My understanding is that condenser/capacitor is there to save points and make time constant of primary circuit smaller for faster feild collapse and stronger secondary circuit voltage. My first guess would be that would not make a big diff on mag drop.

I would guess the coil could be more likely but would expect that it might be more related to mag temp and resultant expansion and opening of wires in the coil and then secondly rpm or manifold/cylinder pressure effects from decreased spark voltage. Your symptons might match this scenerio.

But let me ask a question, does it skip/miss/pop as if just one cylinder is having a problem or does it appear that all cylinders are effected?

Dave

[Guest]

Re: Mag Question

BTW- Fresno Airparts has the coils for $97.50, have other parts too. Dave

[Acroeric]

Re: Mag Question

We'll- we have just been experiencing a very similar issue with an airplane that came out of a beautiful restoration. We have the same mags. . . Bendix SF4R's and it does fine at idle- in fact it does fine for about the first 10 minutes you run it but after that it starts to miss a bit at higher rpm's. Return to a lower throttle setting and it runs fine. So far we have removed and replaced almost every part in the mags and had the rotating magnet re-magnetized. I will check spark plug gap again. Pulling carb heat makes it just a little bit better but doesn't clear it up. Mags, carb, and engine have all just been overhauled.

Static run-up with a Wood 72-42 Sensenich seems a bit low at 1950 RPM but I am used to the C-85 with the climb prop.

Oil temp and pressure are perfect- fuel burn seems about right on the 3 flights I have made so far.

Any ideas?

[3Dreaming]

Re: Mag Question

We'll- we have just been experiencing a very similar issue with an airplane that came out of a beautiful restoration. We have the same mags. . . Bendix SF4R's and it does fine at idle- in fact it does fine for about the first 10 minutes you run it but after that it starts to miss a bit at higher rpm's. Return to a lower throttle setting and it runs fine. So far we have removed and replaced almost every part in the mags and had the rotating magnet re-magnetized. I will check spark plug gap again. Pulling carb heat makes it just a little bit better but doesn't clear it up. Mags, carb, and engine have all just been overhauled.

Static run-up with a Wood 72-42 Sensenich seems a bit low at 1950 RPM but I am used to the C-85 with the climb prop.

Oil temp and pressure are perfect- fuel burn seems about right on the 3 flights I have made so far.

Any ideas?

Dad's airplane did that when it was first restored. As it warmed up first 1 mag would be rough, then they both would get rough. After about 30 minutes of run time they would check OK. Then came a stuck "VALVE". After we fixed the stuck valve we started using some MMO. Mag problem went away. The valves must have been hanging slightly as it come up[ to temperature. Tom

[Tom G]

Re: Mag Question

Thanks everyone for your ideas.

Dave, to answer your question it does appear that it is effecting all cylinders. There were some times when it felt like a steady miss like it was only one but the other day it was all of them. Thanks,

[Tom G]

Re: Mag Problem Solved

I just got back from the airport. today we decided to check out the plugs and found out that when my ercoupe cups were installed the gasket was left on the spark plugs. As you can imaging that kept quite a bit of the plug out of where it belonged. After removing the gaskets from the cups (which wasn't easy) it ran fine. What a relief!