Burning Rotors? - Magneto Question

[Trent M]

Its a good thing to fix the timing like that, just so everything is right. You may have solved a lot of the problem by getting the timing right, it could have been off some. For a tractor that is used that much for that long, I'd expect the points and condenser are ready for replacement. My guess is your problems are likely solved when you receive/replace those. I generally at least dress the points in my mowing tractor every other year.

[Radec Aksarben]

Well, the cub mowed with flying colors. I didn't change the points yet, but that is on the less urgent to do list now.

Here it is ready for a well deserved bath. (My wife picked that wildflower bouquet for me sticking in the choke loop )

[Hengy]

Nice looking cub, Nebraska Cedar!!!

[outdoors4evr]

Do magnetos fire the spark when the rotor moves or when it is stationary?

What does this mean? Would you please describe this further?

While the engine is turning over the rotor should rotate smoothly without any pause or hesitation. If it does, then better start checking the mag and the governor gearsets.

An engine shouldn't lose its timing. If the distributor slipped a tooth or two, then the gears are not meshing well. Also, check the hold-down bolt to make sure the mag assembly can't turn (allowing it to get out of time).

[Radec Aksarben]

Do magnetos fire the spark when the rotor moves or when it is stationary?

What does this mean? Would you please describe this further?

While the engine is turning over the rotor should rotate smoothly without any pause or hesitation. If it does, then better start checking the mag and the governor gearsets.

An engine shouldn't lose its timing. If the distributor slipped a tooth or two, then the gears are not meshing well. Also, check the hold-down bolt to make sure the mag assembly can't turn (allowing it to get out of time).

The rotor on a magneto moves in impulses. If you have a magneto tractor, have someone crank the engine slowly with the distributor cap off. The rotor will jump ahead 4 times to complete a revolution. I imagine when the engine is running it would smooth out since the process happens faster, but I'm just guessing. My question was based on that observation. I was asking if the spark happened during the impulse or during the "dwell" time. Hopefully someone will clarify or correct me if I'm wrong on this.

I'm assuming whenever the rotor was changed last that the Magneto to rotor timing got messed up. When we restored the tractor we were learning as we went along and must have just positioned everything to get it to run. I think that during the years following, the spark had to leap off the corner of the rotor to the distributor to fire the engine because the timing was a bit off (this is based on marks on the new rotor and cap mentioned at the beginning of the thread). Eventually the combination of aging spark plugs, rotor, distributor cap, and points with the poorly timed mag caught up to me and the engine quit working because there was too much resistance in the system, especially when the engine got hot.

At the moment, the new plugs, rotor, cap and proper mag timing are enough to get a good spark. I just need to get around to changing the points.

Thanks again everybody for the guidance.

[John *.?-!.* cub owner]

A slight addition to the above statement. At cranking speed, hand crank or starter, the rotor does move in jumps due to the impulse loading a spring and releasing it to spin the magnet in the magneto faster and get a better spark. At running speed, the impulse disengages, and the rotor turns smoothly with no jumps. Under both conditions however, it is moving when the spark occurs and is transmitted to the spark plug wires.