J-3 ignition noise

[bob turner]

J-3 - no generator, just a battery and a couple of radios. All wiring includes ground wires back to a common point.

Always had a little ignition noise since converting from Slicks to Bendix (S4 LN-21). Started to bother me in May last year - pulled both mags and replaced with OH units. Fixed one spark plug lead, cured that pulsing noise. Decided I wanted to clear up the background noise, worse on the left mag.

Aug '14 - swapped harnesses around a bit, trying to get noise to move and/or diminish. Moved left harness to right mag, noise stayed mostly on left. Moved the harness back, and made a new one for the right mag. Pulled newly OH/d mag off the left, replaced with the still serviceable old one. No change.

Sept '14 - replaced left mag with OH/d unit. No change. Feb '15 - installed $140 worth of new filter capacitors withnproperly grounded shields. No change. Installed new left harness. No change.

Plugs new in August. Ignition switch replaced with toggles. Thought it helped, but no . . .

trial baloon - P-lead on left replaced with new wire, shield, and toggle, draped over front seat and stuck in seat pocket. No change.

Noise is getting worse - left mag drives Icom portable on internal batteries and whip antenna nuts! Both mags present some noise in the Garmin SL-40. Tower is not complaining, but it is really starting to bother me.

Tomorrow I shall change the left plugs ( currently new Champ shorties), test, then run a test with P-leads totally disconnected.

Is it possible that the internal capacitors on three of my mags are all bad at the same time? Should I swap left and right mags next?

I use shielded P- leads, and use the shield as a ground when the mags are off. That means when mags are on, the shield is grounded only at the mag itself.

Any hints are welcome.

 

[qsmx440]

Well the normal suspects have not showed up so a couple of questions: Your calling this "background noise"? So is it covering signals or just slightly opening the squelch and being annoying? There is usually some noise from an ignition but should not open the squelch or interfere much with legitimate signals. The second thing you said is "handheld on rubber duck" so this is a situation where you are attenuating the outside signals coming in through less than wavelength openings in the metal so any normal noise inside the cockpit would have more effect. Have you tried the handheld in a similar aircraft? Just curious about the intensity of the problem. If it's not bothering the signals ("background noise") could you tighten the squelch a little and solve your problem?

 

[bob turner]

It is not breaking squelch. Only on receiving - it is all the plugs, and varies with RPM. The "other" Cub has an identical portable setup ( no Garmin, though) and Eisemann mags. I made both harnesses on that one - no noise at all.

Until yesterday, very close and strong signals were really quite clear, especially on right mag only. I might put the old harness back on - it is Champ red wire. New one is Packard 440, and I have made a lot of them without problems.

 

[qsmx440]

It sounds like it is real interference so should make it easier to find. I can't address the mags or specifics of the aircraft equipment but a couple of stratigies that have worked for me when trying to pin point this kind of thing in ambulances and patrol cars:
First find a weak steady signal, say a distant atis. With the aircraft secured and your wits about you to keep from being decapitated or losing an arm, use your portable as an indicator of improved conditions, use a battery type jumper cable from the negative battery lead and touch the various "noise" antennas in the engine compartment. Things like exhaust parts, p lead shields, engine case, mount etc. Anything that should be at ground potential. With a small capacitor in series with your test ground you can touch the hot things like starter cable etc.
You asked for suggestions so maybe this would help save you from expending money trying to find it.

 

[bob turner]

Thanks. This is a no electric airplane - it is just ignition noise. It is definitely worse on left mag, and it is not the P-lead, verified this morning by swapping P-leads at the capacitor.

Pulled the brand new shorty Champions out of the lower cylinders (all on left mag) and replaced with known good standard length Tempest plugs - dramatic noise reduction, but still noisy. Tomorrow, old harness goes on, capacitors get swapped, and finally old right mag will go in left mag slot.

 

[Gordon Misch]

Bob, this is just a shot in the dark by a blind man, but - - Resistor vs non-resistor plugs, and/or leads? Good connection between leads and plugs? Maybe some sort of breakdown happening in the high voltage (plug) circuit?

 

[Ruffair]

Radio noise in a J Three....!

might be a first....!!

 

[N86250]

Could it be the intercom or its wiring is going sour or it needs a fresh battery? You didn't say you used one so it's a wild a$$ed guess. jrh

 

[skywagon8a]

Bob, You have moved, swapped, replaced, altered everything ignition. Yet the noise is still present.

Is there corrosion growing on your ground connections to the radio power source?

Antenna(s), Fixed to the portable radio or remote to a separate antenna mounted somewhere? Corrosion again on these connections?

What happens when you move the antenna to a different location/relationship to the engine?

Did you try moving the radio to the quiet Cub? What happened?

 

[bob turner]

The last test was with the aircraft battery removed and a hand- held with battery and rubber ducky antenna. And the left mag is still the culprit. I changed spark plugs, and got what I thought was a dramatic reduction in noise, then a lot of it came back. It is better now by a factor of maybe two.

I am very close to simply swapping mags, to see if the noise migrates to the right. I have swapped everything else multiple times.

 

[iancallier]

Have you thought of moving home to a place where you don't need the wireless?

 

[bob turner]

Yeah - the Super Decathlon will need either a move to Montana or ADS-B in five years.

I spent my first seven years without a radio of any kind. I would not be without one now, and I won't leave home without a wing tank. Aviation has changed since I landed in that cow patch that is now under Dulles International Airport.

 

[wireweinie]

Bob
Make sure there are no airframe grounds on the P-lead circuits. The only grounds on the 'G' tab on the key switch, or the ground tab on a toggle, should be a small jumper to the shield of the shielded P-lead wire.

Also check the routing and security of the ground and power leads for the avionics. Make sure they, and the coax for an external antenna, are as far away from the P-leads as possible. Where is your avionics ground, now? If nothing else works, ground your avionics directly to the battery.

Check for a ground strap from the airframe to the engine crankcase. Good idea even without a starter.

Check over all the small details. If it was something big you would have found it by now.

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[bob turner]

Thanks. Was hoping you would check in. P-leads are out - swapped left for right and the noise stayed on the left mag. So my next step is to swap mags - left for right. I do not use airframe ground - even the lights have two wires. I do use buses in the wing root, but again, the P-leads are not the source.

Just in case, I will swap the brand new capacitors and the left mag harness yet again - that is easier than pulling both mags, but as you know, in a J-3 everything is more or less simple.

 

[wireweinie]

Just a thought. You mentioned capacitors. Can you describe where they are located? On the mag or on the firewall? And let me know what kind of antenna setup you have and if you have a ground strap from the engine to the airframe. If you want just contact me direct and I'll give you my cell number.

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[bob turner]

No ground strap. Capacitors are the $70 ones from Spruce - attached to the back of the mag, with shield grounded at the screw-on cap. Brand new. Noise was present before and after installation. Old capacitors were bolted to engine - more convenient.

I would think antenna ground would be affected equally by both mags, but since I effectively remove all internally grounded radios when I take the battery out, I sort of assume that there are no resonant circuits. If something without power is resonating with spark plug noise, I would think it would resonate with both mags.

Stearman makes the same noise, but there we do have starters, generators, ground straps, etc. it just never bothers folks much. My airplane is completely functional; it is just bugging me that it stays on the left mag no matter what.

I will swap mags before I post again. That's the only part I have not swapped from right to left. Noise is always on the left, with swapped plugs, wires, capacitors, P-leads, all in separate trials.

 

[wireweinie]

No airframe ground for the avionics protects them as well as you can. But antenna location can have some bearing on the problem. And you're right about the left system being the problem. Something is causing it to radiate the noise. Put the ground strap on the engine and then swap side with the mags. Strap is easier of the two. If the noise changes with either of the two choices, it tells you the source.

Let us know what you find.

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