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Failed mag on B&S single in riding Murray?


Fred_C_Dobbs's Avatar
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09-11-12, 06:00 PM   #1  
Failed mag on B&S single in riding Murray?

Please excuse the tangled narrative, but I don't know enough to know if any of this is significant ...or not.

I've mowed my Mom's yard all year with her 12-hp B&S single riding mower. On cool mornings it needed a shot of starter fluid to get it going but seemed to run fine thereafter. Once it was warm it usually restarted without the ether. The plug always read okay, the insulator a nice cinnamon brown.

When I mowed with it last week, I installed a new plug to see if it would help the hard starting. It didn't. In fact, the last half hour I ran it, it kept stalling almost to the point of dying, then it would kick back in, like I'd turned the ignition off and then switched it back on. This would happen every minute or so, and I couldn't say the stumble was linked to any movement of the mower, turning or stopping or whatnot. No backfiring or anything, just die for a fraction of a second, then fire back up.

This mower is stored in a large weathertight shed but we've had a whole lots of rain here, courtesy of Hurricane Isaac, which got me to thinking all the humidity might have left a few drops of water in the gas. So when I finished the mowing I drained the carb, and checked a sample of the fuel (no sign of water in it). That was when I noticed the intake manifold was loose where it bolts to the side of the cylinder block.

This big single has shown an annoying tendency to shake loose different pieces parts, and the intake manifold has come loose before. But it didn't act like this. Before, it didn't make good RPMs, the exhaust note sounded funny, and when it quit, it quit pretty emphatically in a great puff of gray smoke. Anyway, after draining the gas, I remounted the carb as per normal.

So yesterday it wouldn't start, never heard it hit, and the ether didn't help. I went back to the most recent spark plug -- the one in use until last week -- but no dice. I'm pretty sure I've got air and fuel but I couldn't see any spark. However, I'm a famously poor mechanic, and I'd never tried before on this mower to look for spark (meaning I'm not sure I'd have seen it if it did spark).

Anyway, I pulled the cowling, removed the magneto, ran a wire brush over all the areas of contact where it mates to the mower, remounted it a matchbook cover's thickness from the flywheel, and checked for spark again. Still nothing, even with the cowling off to give me free and easy access to bare cylinder fins for grounding. To make sure my eyes weren't fooling me, I even checked it again about sundown, when it was purt near dark in the shed. Couldn't see a thang.

Now if I were any kind of mechanic, at this point I would pronounce the magneto dead and go buy a new one. But I ain't no kind of mechanic, so I don't have much confidence in my diagnosis. So the reason I've written all this is so ask whether any of the problems what I was experiencing before the complete failure (hard to start when cold but runs okay when warm) is consistent with a failing magneto. Do they sometimes fail gradually? What about the intermittent sputtering the the last time I was mowing? Does that sound magneto-ish, or could the loose carb have been doing that?

Thanks in advance for the help!

 
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cheese's Avatar
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09-11-12, 06:36 PM   #2  
The coil can fail many different ways, often in the manner you described. Make sure you put the coil back on right side up, then unplug the kill wire from the coil. If you have no spark at that point, the coil is bad, replace it.


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

God bless!

 
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09-11-12, 09:14 PM   #3  
Stupid question, but it only has one wire attached to it (but it also makes electrical contact through the two mounting points). So I guess that one wire has to be the kill wire, yes?

 
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09-11-12, 11:00 PM   #4  
yes
_______________________________________


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

God bless!

 
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09-12-12, 09:54 AM   #5  
Bada-bing! Dropped in the new mag -- $40(!!!) at the Co-Op -- and it had a beautiful, bright blue spark. Put it back together and it fired right up and ran like I'd given it a dose of electric viagra.

Thank you, cheese, you're the ...cheesiest.



(sorry, couldn't resist)

 
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09-12-12, 09:56 AM   #6  
Lol, no problem, glad I could help!


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

God bless!

 
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