Magneto Timing

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NunavutPA-12
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Magneto Timing

#1 Post by NunavutPA-12 » Mon Sep 26, 2016 1:16 pm

Home-built modified PA-12 clone. AME not easily accessible - I'm in Kugluktuk, nearest is in Yellowknife, 375-miles distant. Otherwise I'd get a pro to do this.

I got my right mag back from a 500-hour inspection and repair. Now I have to time it to the engine. I have the tools but this will be the first time I've attempted this particular job. I think I have figured out how to find TDC on the compression stroke, etc. thanks to YouTube (!).

Obviously, the mag itself has to be in a certain position prior to installing it on the engine, and I'm not too clear on how this is done. So I need some advice.

It's a Bendix mag on an O-320B2B.

Thanks.
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Glasnost
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Re: Magneto Timing

#2 Post by Glasnost » Mon Sep 26, 2016 3:10 pm

Set the engine to the correct advance. TDC Compression,, should be a mark on the flywheel that lines up with the case split on the back or the dot on the starter on the front about 25 degrees advanced. Leads off and one set of plugs out for easy turnin

Take the plastic cap off the mag. If you don't see a white gear, try the other cap. Turn it backwards until the red tooth is in the middle and you feel it stick. Or turn it forwards if you like snapping impulse couplings and getting shocked.

Put it on,, making sure the gear don't move from the sticky spot, snug it up finger tight, do the same for the other side and don't forget the gasket. Hook up your mag timer (mags on or off don't matter if p-leads are removed). Rotate mags so the gear goes opposite direction of rotation, then the same until the light just goes on (tap with palm of your hand) snug nuts with 1/2 inch. Same for other side. If theirs an impulse coupling, make sure it's snapped through or it will throw everything off.

Then back off the prop a few degrees and slowly bump it up until 25 (the backing each time is so you can take out the gear backlash) and check to see if both lights come on at same time at proper time. Do final torque, connect p leads, connect harness. install plugs and leads. Run 'er up to see if it will start, mag drops good, no oil leaks etc then close her up.
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NunavutPA-12
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Re: Magneto Timing

#3 Post by NunavutPA-12 » Mon Sep 26, 2016 4:19 pm

Okay. Thanks for taking the time to spell that out. I think I have the theory down pat - now it's time to actually do it.

Since I have to work outside I'm waiting for one reasonably nice day with temps above zero and no wind.
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Choppermech1986
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Re: Magneto Timing

#4 Post by Choppermech1986 » Tue Sep 27, 2016 2:29 pm

Having done that job numerous times, I'm grateful to have learned under the tutelage of experienced engineers in well above zero temperatures. All the best with the job!
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