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My walk behind mower wont start..has no spark with new plug or old one could it be a rusty magneto

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  • rattosh51 May 11, 2010

    What brand lawnmower is it? Some brands have a coil that goes out (Lawnboy)

  • bulldogchief
    bulldogchief May 11, 2010

    maybe, how did you check for spark?



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Rust on the magneto has no bearing on the output of a spark. The magneto or the points if it has them could be the problem. If the unit has points they may be under the flywheel. If so equiped, that would be the logical place to start. If no points, I would replace the magneto. When you place the new one on, place a piece of paper between the flywheel and the magneto. When you tighten the magneto, pull out the paper and turn the flywheel by hand. If the magneto turns without touching, put everything together and try it out. Good luck...Lee

Posted on Sep 09, 2009


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Need gap specifications

The gap between the flywheel and magneto (pick-up coil) is not that critical, if your engine is not running right or won't start. 2 to 3 mm is fine. The key point is that the gap be uniform in the arc where the fly wheel passes it. Please post your symptoms and we can better assist you on the forum. If you are not getting any spark from the plug wire... check this at night by unplugging the plug boot and pulling the cord and watching; if you can't detect a spark... then you have a magneto issue or a bad spark plug wire. A little crokus cloth to rub the rust off the entire flywheel edge surface will help. Don't sand down or apply solvents to the magneto surface even if it's rusty!

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The engine stops after a half hour of mowing and will not start until it cools down, after which the cycle repeats itself

engine getting hot. Take rope pull off and clean off all the air fins on the cylinder block, use air how and do a good cleaning. If flywheel still have all the cooling fins it should help the problem or even solve it. Keep mower airways clean.

Jun 05, 2014 | Tecumseh Lawn Boy Push Mowers Insight...

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Why wont my walk behind mower wont start

take your spark plug out put some gas in the chamber replace plug if it fires and not run it is in fuel delivfry check and see if carb. is gummed up if so clean it if not check for restriction in gsas from tank to carb. one of these is the problem

May 23, 2014 | Tecumseh Lawn Boy Push Mowers Insight...

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Have a DR walk behind bush hog.changed magneto,spark plug ,has good spark and is gitting gas.I cleaned the carb it turns over but will not even try to start.

Are you really sure it's getting gas. Is the Plug wet after trying to start?
Did you clean the Jets not just clean out the bowl?

Sep 08, 2013 | Garden

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Briggs engine 143-586244 on a old snow blower will not start. Evan when I use starting fluid, choke it or open very little. It has not ran for about 30 years. I drained the old gas back then. Now I...

Do you have spark at the plug.if in storage that long its possible the coil and magneto clearances are corroded. I get alot of mowers in that have been stored outside over winter that are rusty and corroded.

Aug 29, 2011 | Garden

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875 Series Engine-cleaned flywheel and magnets and magneto. Put back together and now no spark to spark plug. What is the air gap between the magneto and flywheel.

You should put it at .020

Are you sure you had spark before you took it apart? If you are not sure, then the Ignition Coil may have went bad. How did it act before you decided to take it apart?


Aug 17, 2011 | Briggs & Stratton 190cc 875 Series OHV...

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Have a very old tractor that the magneto isn't sparking

another thing you should check is see if the flywheel key is in good shape even if there is a small nick in the key that would prevent it from sending a spark to the plug also check the magnets on the flywheel if they are rusty that would act the same way. sand the magnets just untill the rust is gone(don't rub to hard the last thing you want to do is remove to much of the magnet then you'll have to regap the magneto)....good luck

Apr 12, 2010 | Garden

1 Answer

Riding mower wont start

Hi Pry

The spark comes from the Magneto. Follow the spark plug wire and you will find the magneto at the end of it

The first thing you should check though, is the safety kill switch. There is a pressure switch in the seat that kills the engine in the event the driver falls off.

You need to be sitting on the mower to start it.

If the switch is ok and you still get no fire, it's the magneto.

Best regards,

Sep 07, 2009 | Garden

1 Answer


Not so many reasons she won't start. 4-Strokes are Simple!

Here they are:

Fuel Tank-> Fuel Line-> Fuel Filter-> Carb-> Intake (Valve)-> Piston-> Exhaust (Valve)-> Muffler

Carb (Old Style):
Fuel Side:
Fuel Bowl-> Float-> Needle Seat-> Fuel Ports (Venturi)
Air Side:
Air Filter-> Carb Air Intake-> Butterflies (2)
(Clean 'em ALL!)

Hardware (Motor):
Rotor (Magnets)-> Magneto (Ignition Coil)-> Spark Plug Wire-> Spark Plug-> Cylinder Head (Spark Plug Ground)

Kill Switches (Seat, Clutch, etc.)-> Spark Plug/Ignition Coil-> Ground
Key Ignition-> Battery->Starter/Generator

If one of those fail.......= Non-Starting 4-Stroke!

You are either getting too much, or too little. Too much = Carb.
Too little = Carb. Therefore, tear down the Carb (COMPLETELY-Careful!) and clean it up.

Grounding out the System (Kill Switches)
Battery-> Ignition-> Starter.
Magneto->Plug Wire->Plug (Check 'em all!)

May 28, 2009 | Mtd Garden

1 Answer

It wont spark at all. No fire. Just put a new spark plug so that's not the issue. This lawnmower is only 1.5 years old. Thanks

Could be magneto bad, wire from magneto bad, ground on magneto bad, or the clearance between the flywheel and the magneto is rusted up if it was outside. Clean flywheel with sandpaper and set magneto gap using a match book flap.

May 04, 2009 | Toro Lawn Mower With Toro Power Tools

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