Ignition help needed

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bifft

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So, I've documented a little of what I'm now calling The Neverending Condition Inspection over on another thread, but now I'm stuck. I sent in my magneto for a mandatory service bulletin, they sent me back a rebuilt one. When I install the rebuilt one on the plane, it works just the same as if that magneto isn't there at all.

I run one Slick magneto and one P-mag on a O-360. I start on the P-mag (the magneto is a non-impulse model) and that works normally. I go up to 1800 RPM, then switch the magneto off. No RPM change. Switch off the P-mag and the engine stops. If I turn it back on again before the engine completely comes to a stop it runs again, with a big "bang" (presumably unburned fuel in the exhaust).

Before I installed the mag, I checked the harness with a ohmmeter, all the lines tested out 0 ohms,. Checked that the harness is going to the right plug, the cylinder lines up with the number printed on the nut. Checked the timing again, the "buzz box" switches on right as it passes the 25 degree mark on the flywheel. I checked a continuity tester against my switch, it is grounded when the magneto is "off" and open when "On". The continuity tester does show a connection between the p-lead and the case, but from what I read this is normal.

I took the magneto back off, put it on the bench with the plugs installed and the outside wired to the ground plug. Rigged up a little block to spin it with my drill and all four plugs spark well. Not as big of a spark as I got out of the P-mag back when I tested that, but clearly sparking. I can even get it to spark most of the time just turning the gear by hand.

Is there anything else I can do other than just put it on the engine again? I guess I should gap the plugs again, but I just did that as part of The Neverending Condition Inspection.
 

TFF

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The first thing I would be checking is if the points were clean. They can come with some type of preservative and can make terrible contact when new. I use a business card and a shot of contact cleaner to buff them. Of course you can see if they are opening too. It was timed to the engine correctly I assume. If the P lead is shielded, make sure none is grounding the mag, I had that happen once. Ok that would be my first check, probably engine timing next , then points.
 

TerryM76

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Have you tried running it with the P-lead disconnected? I’ve done that to isolate an ignition problem in the past. Just be very cautious since you will have a “hot mag”.
 

wsimpso1

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First thing occurred to me is if the mag is timed to fire on the wrong turn of the engine. That is one half turn out at the mag but one full turn out on the crank, at top of exhaust stroke instead of top of compression stroke. Called a waste spark, and would fully account for behavior cited.
 

TFF

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If it was 2-3 teeth retarded, it wouldn’t do anything. 2-3 teeth advanced and it would be a big problem as would 180 out. I’m not a fan of Slicks and having to pins to time. Pins get forgotten. Mine has slicks but my preference is the Bendix.
 

Dan Thomas

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The continuity tester does show a connection between the p-lead and the case, but from what I read this is normal.
It's only normal if the points are closed. Rotate the mag and see if the resistance goes up a few ohms. Just a few.
 

proppastie

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Pull plugs ....disconnect p lead .....set plugs on engine and spin prop by hand
See if you have spark. ....see where on the rotation (timing ) and if on compression stroke. No spark does not make sense because had it on bench. Reconnect p lead If not spark then switch problem. It is easy to put slick pin in wrong hole or not on number 1 compression .
 
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bifft

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Thanks, this has given me a lot of things to try. The resistance does vary as you rotate the gear.

One of the reasons it is taking so long is that I can only get to the airport in daylight on the weekends this time of year. I'll give it another go Saturday.
 

Dan Thomas

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Thanks, this has given me a lot of things to try. The resistance does vary as you rotate the gear.
Rotate it slowly. Real slowly. That rotating magnet causes current flow in the primary that will mess up your resistance readings.
 

saabnut

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Sep 10, 2021
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So, I've documented a little of what I'm now calling The Neverending Condition Inspection over on another thread, but now I'm stuck. I sent in my magneto for a mandatory service bulletin, they sent me back a rebuilt one. When I install the rebuilt one on the plane, it works just the same as if that magneto isn't there at all.

I run one Slick magneto and one P-mag on a O-360. I start on the P-mag (the magneto is a non-impulse model) and that works normally. I go up to 1800 RPM, then switch the magneto off. No RPM change. Switch off the P-mag and the engine stops. If I turn it back on again before the engine completely comes to a stop it runs again, with a big "bang" (presumably unburned fuel in the exhaust).

Before I installed the mag, I checked the harness with a ohmmeter, all the lines tested out 0 ohms,. Checked that the harness is going to the right plug, the cylinder lines up with the number printed on the nut. Checked the timing again, the "buzz box" switches on right as it passes the 25 degree mark on the flywheel. I checked a continuity tester against my switch, it is grounded when the magneto is "off" and open when "On". The continuity tester does show a connection between the p-lead and the case, but from what I read this is normal.

I took the magneto back off, put it on the bench with the plugs installed and the outside wired to the ground plug. Rigged up a little block to spin it with my drill and all four plugs spark well. Not as big of a spark as I got out of the P-mag back when I tested that, but clearly sparking. I can even get it to spark most of the time just turning the gear by hand.

Is there anything else I can do other than just put it on the engine again? I guess I should gap the plugs again, but I just did that as part of The Neverending Condition Inspection.
If it sparks turning by hand, I would say the mag is in good condition. I think the rotor is indexed wrong. When you installed it, I assume you used the pin, in the appropriate hole? L/R/X This corresponds to the magneto label for rotation. X is for LASAR.
 

Dan Thomas

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If it sparks turning by hand, I would say the mag is in good condition. I think the rotor is indexed wrong. When you installed it, I assume you used the pin, in the appropriate hole? L/R/X This corresponds to the magneto label for rotation. X is for LASAR.
Maybe he even has the wrong rotation for his engine.
 

bifft

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I assume you used the pin, in the appropriate hole?
Used the "L" hole as it was a "L" mag. This is also how I set up the old one which worked fine. It is going into the right hand hole (viewed from the back of the engine).
 

bifft

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Had to swap on call weekend with a coworker, so didn't get to it until this last weekend. Carefully reinstalled it, checked the wires, Made sure to time it on the compression stroke. Put on the plugs and set them on the engine so they would be grounded. Turned the prop as fast as I could. No spark. Tried it with the starter motor instead of by hand. No spark. Tried with the p-lead disconnected, no difference.

Not quite ready to give up, decided to put the plugs in and just try running it. Ran like a top. Maybe 75rpm drop switching off the p-mag,. I guess with no impulse coupling it didn't spin fast enough to spark by hand or with the starter. My best guess is that I had previously timed it to the exhaust stroke.

Sadly, by that time I was tired and cold, the weather wasn't great. Decided to put off flying for some other time.

Thanks for all the help.
 
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Dan Thomas

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I guess with no impulse coupling it didn't spin fast enough to spark by hand or with the starter.
That was part of it. The other part was likely being 360° off, firing on the exhaust stroke like you say.
 
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