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I got a free string trimmer from a buddy who couldn't get it to start.

I did a compression check by pulling the pull starter and the compression seemed good. Next I pulled the spark plug, put it back on the boot, grounded it to something metal. I then pulled the cord again and saw no spark. I kept doing this and some times I'd get a very weak spark, but most of the time I'd see no spark at all.

I haven't had time to look at it again for a few days, but I will be pulling it out again probably tomorrow. What things would you check and in what order?

Niall C.
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mikeazo
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4 Answers4

7

Use a Spark tester (which can be found at any automotive supply store), to test the spark.

Spark Tester

To use it:

  1. Remove the boot from the spark plug on the trimmer.
  2. Plug the end of the tester with the metal probe into the boot.
  3. Connect the other end of the tester to the spark plug.
  4. Give the trimmer a couple pulls, while watching the testers spark chamber.

No Spark

If you don't have a spark, you probably have to replace the ignition coil/module. There's a good YouTube video that shows how to replace a lawn trimmer ignition coil. Your model may vary, but the procedure will be similar.

Weak Spark

If you have a weak spark, try cleaning, regapping, or replacing the spark plug.

How a trimmer ignition system works

The ignition system is made up of 3 major parts. A magnet (located on the flywheel), an ignition coil, and a spark plug. When you turn the engine (pull the cord), the flywheel rotates. When the magnet on the flywheel spins past the ignition coil, a current is induced. This current travels through the ignition wire to the spark plug. The electrons want so badly to return home (ground), they jump the gap in the spark plug. As they jump, they produce an electric arc (also known as a spark). This arc ignites the fuel air mixture in the chamber, and the trimmer starts.

Community
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Tester101
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On top of Tester's excellent answer, here are some quick and easy ways to get it working without going through the trouble of replacing the ignition system or buying special diagnostic tools.

Clean the Spark Plug

Take a bit of fine sandpaper and try to rub the grime and dirt off of the spark plug. Give it a little bit of a shine and try again. You can also try a small amount of carb cleaner, but just let it dry out before attempting to start it again.

Clean the magneto

Over time small rust spots can form on the magnet in the ignition system. It may not look like much but this rust can greatly interfere with the electrical current generation needed to form a spark. This can also be fixed by taking fine sandpaper and rubbing the rust spots off of the magnet.

maple_shaft
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You can test the spark using a short test lead with alligator clips. Remove the spark plug, reconnect the plug wire and using the test lead, connect the plug ground electrode to a good engine ground point. Turn on the ignition switch and pull the starting cord quickly. You should see a spark in the plug gap. Since the spark may be faint, it may help to do the test in a dark area. If there is no spark, check for a broken or shorted (bare) lead wire or bad switch. I've had to replace the switch on my edger twice. It uses the same switch as many other edgers and trimmers. If it's not a lead wire or switch, replace the coil.

Gary Green
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just had similar problem on a pals 240, turned out to be a bare wire going to the switch, very easy sort!

sandy
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