FM-J Magneto Timing

Paul in CT · May 3, 2012

I just had my Fairbanks Morse FM-J magneto rebuilt by Mainely Magnetos(it looks great!). When I took it off of the engine (McCormick Deering 10-20) to send to them I removed the cover and made sure the the rotor was pointing to (just before) the #1 plug wire. I am making the assumption that the #1 piston is near TDC. How do I time the "new" magneto to the engine? It also says to never spin the magneto without giving the spark a place to go. Does that mean I need to hook up the spark plugs before I spin it? Engine was running great and then the condenser and points went and I sent it out to be rebuilt. Any help would be appreciated. I have never had to time a magneto before and since this is an automatic impulse I want to make sure I get it right.

Thanks

old · May 3, 2012

I have the manual on the computer for that mag so if you send me an e-mail I will be happy to send you that manual which might help you

CHUCK CAMBIS · May 3, 2012

correct! never impulse a mag without a tension wire to ground (or sparkplugs). it should work easier by just turning the mag backwards (no spark). you have a 50% chance of having it correct without looking, so bolt it on and if it don't go just remove and rotate one half turn. try again. if this doesn't work, don't pay my bill!!!

El Toro · May 3, 2012

If the engine hasn't been cranked since pulling the mag, point the rotor to same position and reinstall the mag. Hal

CHUCK CAMBIS · May 3, 2012

shouldn't the 10-20 have an ihc mag with adjustable spark advance? i probably have one if interested. the mag is a base mount correct? someone posts up a pic or the numbers i could check for sure.

Tom Fleming · May 3, 2012

Chuck, good practice on the grounding, however the IHC mags have a safety gap built into both the E4A and F4 mags that takes into account the fact that a plug wire MAY fall off during rough operation. This safety gap prevents backflow to the coil and gives the energy generated as the field collapse occurs to prevent coil damage. It is normally set to a greater gap than the plug gap. So, if you have REALLY worn out plugs, or a broken/loose plug wire, no damage to the coil becaue the spark will jump the safety gap.

I cannot comment on a FM mag, but I would think they have the same type of safety feature built in if a wire comes loose from a plug.

while an excellent point, not required for correctly "tuned" IHC mags.

rustred · May 3, 2012

the early ones have the e4 mag with manual advance and later 10-20's have the f4 international mag.

FM-J Magneto Timing

Paul in CT

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old

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CHUCK CAMBIS

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El Toro

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CHUCK CAMBIS

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Tom Fleming

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rustred

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