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MTD leaf vac / b&s engine: no spark at plug


CycleZen's Avatar
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09-02-12, 12:38 PM   #1  
MTD leaf vac / b&s engine: no spark at plug

I picked up an MTD 24a-0200d000 leaf vac with a B&S 12h802-1777-d1 engine. There's no spark at the plug when I pull the cord, so I spent a lot of time checking the grounding- the ground is a separate wire that runs from the ignition system through a switch at the collector bag mount, and also to the carb to allow shutoff via the throttle.

The ground is good and the spark plug is good (put the questionable plug in a different mower and got spark). I removed the pull rope and used a voltmeter to check for voltage: I put one lead in the spark plug boot and touched the other to ground, then manually ran the magnets through the magneto and got some voltage.

So I'm puzzled: why no spark? Can a faulty magneto deliver some voltage if it is not in good enough shape to run? Any ideas for a next step?

Thanks, and have a great weekend!

 
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cheese's Avatar
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09-02-12, 12:49 PM   #2  
Unplug the ground wire from the coil and see if you get spark. If so, the wiring or a switch is the problem. If no spark, the coil is bad.


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09-02-12, 02:50 PM   #3  
Thanks, Cheese- when you say "coil" is that another word for the armature?

 
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09-02-12, 07:11 PM   #4  
Well, yes I guess, for the purpose of part identification. Not to confuse you, but to explain: The armature is integrated with the coil, however the two are two different things... they are just made into one package and are not separable. If you take the ignition assembly apart and look at each component, there are: an armature, 2 coils, a plug wire, and electronic components (mainly a transistor and capacitor). So, calling it an armature is as incomplete as calling it a coil. The coil or the electronic components failed, not the armature. Briggs and stratton calls it an ignition coil but does mention the armature at times, mainly when talking about the flywheel to armature air gap. In that case, it is the armature you are measuring to, not the coil, so they even call it two different things, depending on the context.


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

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09-08-12, 05:22 AM   #5  
Thanks for a good explanation! I'm kinda figuring things out as I go forward, and I knew I would get the words wrong.

I saw a video on youtube that suggested removing oxidation from the points and condenser, located under the flywheel. Is that true for this model? I tried to block the fan from turning so I could remove the flywheel's retaining nut, but the nut turned without loosening. I'm a little concerned someone has been tinkering with this thing- the linkages from the throttle and choke levers are missing, for instance. This is making me wonder if this whole refit project is going to becoming a case of throwing good money after bad.

 
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09-08-12, 12:21 PM   #6  
Yours doesn't have points. If your flywheel nut is turning without loosening, the nut and crankshaft are stripped. Sounds like someone has messed it up. You're probably looking at buying a crankshaft and taking the engine off, apart, and replacing the crank, gaskets, and torquing the rod bolts back to specs. You might find a good used crank on ebay and chance it.


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09-11-12, 09:51 AM   #7  
Argh- this has gone from a "clean out the carburetor and fire it up" project to way more time intensive than I can manage. I'd love to be able to fuss over this thing, but it's just not worth it, I'm afraid.

Thanks for your help, Cheese!

 
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09-11-12, 06:48 PM   #8  
Anytime!
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