How complicated are Magnetos on 4 stroke lawn mowers?

Question by Kevin: How complicated are Magnetos on 4 stroke lawn mowers?
I have no idea how complicated they are and only trying to get my 4 stroke lawnmower to start again. I have run down the basic elements for the lawn mower like checking for a spark, fuel in tank, and carburetor sludge/powder removal from deposit cup. There was a spark before I tested the lead for a spark with another plug but now there is no spark on both plugs. I removed the starter housing and top engine housing to access the flywheel, coil and that part where the coil has a contact with. That part that revolves when the starter string is pulled. The surface on the part that revolves is slightly rusted with a powdery surface.

I rotated the flywheel and possibly the magnetic area.

Best answer:

Answer by Jack Downs 0421
You clearly have no idea on what you are doing. Just assemble it back together and bring it to a mechanic…you could not possibly make your self more embarrassed in front of the mechanic.

Know better? Leave your own answer in the comments!

2 Comments
  1. Reply
    generalw13 May 27, 2013 at 9:18 am

    Without knowing how old the engine is etc, it’s hard to give an accurate answer.
    Just about anything made within the last 20 years or so has electronic ignition timing built into the magneto. With the electronic ones you don’t have separate points and condensers to worry about.
    When you check for a spark, make sure that the key is on, that any safety kill switches are engaged etc. Another option would be to disconnect a thin black wire coming from the magneto before it goes to whatever ground point is used for the “kill”.
    With that being a pull start model, you won’t be able to rotate the flywheel by hand fast enough to make a spark. You’ll either have to put the pull cord assembly back on or remove the spark plug and use a variable speed drill with a square drive adapter and socket to rotate the crankshaft.
    If you can’t get a spark out of it, then replacement of the magneto is probably going to solve your problem. It’s a pretty easy fix involving a few screws and a feeler gauge. Generally, replacements come with a spec on the spacing between the flywheel magnet (the rusted part on the side of the flywheel) and the edge of the magneto.
    If you do get a spark, your next step will be looking into the fuel delivery system.

  2. Reply
    Jim W May 27, 2013 at 10:09 am

    They are very basic units. The gap setting of the points and the pick up to the coil are the main adjustments that can be made. The timing is set by the key on the shaft. Since you had spark and now have lost it I would first check the shutdown wire. It needs to be open to the block to enable the spark. The next problem place is the condensor is weak. Take the unit to the local service center for a spring tune up. Get a service manual for your engine. If this is a newer unit it may have an electronic spark system. Take it to the local service center.

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