# Craftsman 917.287121 not starting



## fliegendaffe (Aug 28, 2016)

Need a little help. I have a Craftsman 917.287121 that backfired twice today. After each time, my genius wife just shut it off when I have told it happens. Now the tractor will not start and I am trying to trouble shoot it. I just replaced the ignition coil and spark plug just 2 weekends ago. Fuel and air are getting into the chamber. I am getting a voltage reading at the connector from the ignition coil, but no spark or start. I don't want to believe the spark plug is bad after 2 uses of the mower. Any help?


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## FredM (Nov 18, 2015)

the flywheel key has most likely sheared and the timing has retarded, remove the blower housing and then the flywheel retaining nut and washer, check to see if the keyway in the flywheel and crankshaft are aligned.

if the keyway is ok, suspect the new coil or plug, do you still have the old plug, why not try that first, if no start then the coil, I know stranger things have happened and still do.

did you air gap the new coil when this was fitted ?.


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## fliegendaffe (Aug 28, 2016)

I gapped the coil at .011


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## Bill Kapaun (May 8, 2007)

The coil should not have ANY voltage applied to it unless you want to fry it.

The kill wire is grounded to KILL spark, which is generated the same way as cheap walk behind mowers. A magnet passing by the coil.


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## fliegendaffe (Aug 28, 2016)

Bill Kapaun said:


> The coil should not have ANY voltage applied to it unless you want to fry it.
> 
> The kill wire is grounded to KILL spark, which is generated the same way as cheap walk behind mowers. A magnet passing by the coil.


Then how does the spark plug get spark unless the coil generates voltage?


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## jhngardner367 (Apr 5, 2011)

As the flywheel rotates,a magnet,in the flywheel passes the legs of the coil,and causes a collapse of the magnetic field.
This causes an electrical discharge,which goes through the wire windings of the coil,which jumps it up to 50,000+volts,resulting in a spark,at the plug.
NEVER put voltage from a battery ,to the coil,or it fries it.
As Bill said, the small wire,is to ground the coil,not to supply voltage.
If you've put power through it,you've probably destroyed it.


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## fliegendaffe (Aug 28, 2016)

jhngardner367 said:


> As the flywheel rotates,a magnet,in the flywheel passes the legs of the coil,and causes a collapse of the magnetic field.
> This causes an electrical discharge,which goes through the wire windings of the coil,which jumps it up to 50,000+volts,resulting in a spark,at the plug.
> NEVER put voltage from a battery ,to the coil,or it fries it.
> As Bill said, the small wire,is to ground the coil,not to supply voltage.
> If you've put power through it,you've probably destroyed it.


I am aware of how this process works as I am a generator mechanic.


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## Bill Kapaun (May 8, 2007)

You may have a voltage there when the engine is running.
There should be NONE when the engine isn't running.
You stated it won't start, ERGO it's NOT running.
Believe it or not.


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## fliegendaffe (Aug 28, 2016)

Bill Kapaun said:


> You may have a voltage there when the engine is running.
> There should be NONE when the engine isn't running.
> You stated it won't start, ERGO it's NOT running.
> Believe it or not.


Correct and while cranking there will be voltage too. mind blown


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## FredM (Nov 18, 2015)

is there any outcome from this, I suggested checking the key in the flywheel !!, a few drops of water in the fuel will give you this backfire problem also, have you thought of draining the float bowl and checking for condensation ?.


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## fliegendaffe (Aug 28, 2016)

FredM said:


> is there any outcome from this, I suggested checking the key in the flywheel !!, a few drops of water in the fuel will give you this backfire problem also, have you thought of draining the float bowl and checking for condensation ?.


There is no update as I just looked at the key and it is fine. Attached picture.


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