how to check if a mag is really firing???

[7DeltaLima]

I'm sure there is a simple solution but... how can I check to verify that a mag is in fact firing or not?

I flew Young Eagles for about 3 hours last Saturday so was a constant hot start exercise. No problem for all the flight but when I went to start the plane to put it back in the hangar I could not get the plane to start... It's an IO360A1b6 in a RV7A.

Best way to describe it is just hot start that I missed catching on the first attempt. Then thought I'd flooded it, etc. It would hit a lick or two then die. Finally gave up and got a tow to the hangar. Came back a couple days later assuming that the flooding stage was over so I assumed it'd start right up. Still no joy. Pulled plugs. Look good but cleaned/dryed them anyway.

Finally got to the point it wouldn't even hit. Put the buzz timing box on to check mag timing. Left impluse mag light would never stay solidly lit. Basically, showing constantly closed points. It would only blink quickly when pulling the prop through. Right mag showed okay.

Found the Slick mag overhaul manual ... pulled the left mag (first time for that... guess this is the educational part huh...) There seemed to be a spot, maybe the size of a pencil point between the points that was probably keeping the points from opening. Like an arc point.

I sanded the points...set the e-gap according to manual, installed the mag, set the timing..(buzz box showed all okay).... totally excited just knowing that I've solved the problem. No joy. still will not start.

Have fuel flowing. Mag shows timed correctly, 25* bdtc... even remembered to pull through to release the impluse coupler.....

I've ordered a new mag that'll be here tomorrow as I'm still thinking I'm not getting spark. I'll install tomorrow night but before doing so just wondering how would you check to see for sure if you are getting spark or not.

Remember the old days in the farm trucks of pulling the plug and laying it up on the manifold, taking a screwdriver and shorting the starter and watching the sparks jump across the plug to verify spark. Gotta be something that would accomplish the same thing but more eloquently by putting a light between the wire from the mag and the plug... maybe...

any ideas....

thanks.
Doug

[JHines]

I have one of these tester lights:

http://www.amazon.com/Lisle-26900-Sp.../dp/B000CODRMM

Should be able to find one in any car parts store. You put the grooved end over the spark plug wire and clip the ground lead to the engine block. If you're getting spark, the little light will flash.

Don't know why it wouldn't work with a magneto.

[joeboisselle]

I'm sure someone else will try to solve your problem, but to answer your "how to check if you've got fire" question... if the magneto is grounded something (the frame, a metal table etc) and you have a lead within sparking range of the same metal object,(without the sparkplug, it's easier to see and hear) give the mag a spin and you should see a spark once every two revolutions. Also helps if you're spinning it the same direction as it turns while it's running. I've been shocked once while doing this, but I'm still alive to tell the tale. =)

[PaigeHoffart]

One thing to watch out for: It's harder to get a spark to jump a gap in the cylinder under compression than it is at atmospheric pressure. I had a friend with a 172 that had a heck of a time starting; the mags would deliver a spark at atmospheric pressure, but were too weak to fire the plugs at low RPM when under compression. Overhauled the mags and it starts on the first blade.


Paige

[Radomir]

Basically what Joe said, except attach the plug to the lead.. and just rest it on the cylinder.. as you pull the prop through, you'll see the spark... (one safety note: pull all the plugs out).. Also, observe that the mag is timed correctly and firing order. There's no need to get shocked

[osxuser]

Kind of crude, but take hold the harness in your hand while grounding the same hand against the engine. Spin the prop... if you get a shock, the mag fired. It works, we did it in A&P school, kinda dangerous if you let the shock go through your heart, but I've never heard of any ill effects.

[RatMan]

An A&P friend of mine showed me a trick.

He showed that a mag was firing by removing the harness and placing a straightened paperclip in one of the holes that the wires from the harness goes in. He then bent the clip so that it was about 1/4 inch from the mag p-lead and turned the mag by hand. A spark was clearly visible. He repeated this in each hole. All four had the same result. He said it was a good mag.

Now this isn't scientific by any means but it made sense to me. I wouldn't touch the paperclip though.

[Sid Lambert]

hah, I think that was a case of the old guys seeing what they could get the kids to do.

I worked for the county bus shop after school a few years. The guys would often leave charged capacitors sitting on our little break table. I learned that trick on the first shock.

Originally Posted by osxuser

Kind of crude, but take hold the harness in your hand while grounding the same hand against the engine. Spin the prop... if you get a shock, the mag fired. It works, we did it in A&P school, kinda dangerous if you let the shock go through your heart, but I've never heard of any ill effects.