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Unusual no spark problem with TroyBilt Weedeater TB20CS


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08-19-13, 10:22 AM   #1  
Unusual no spark problem with TroyBilt Weedeater TB20CS

I have a TB20CS weedeater (won't start) which has spring-assisted starting on the pull rope. When the spark plug is not in the hole, I get a blue or yellowish-orange spark when I pull on the rope (I have a spark test connected). When the plug is in the hole, no spark. I assume that the flywheel is not being spun quickly enough when plug is in hole.
It has 2 magnets, adjacent to each other, that I have wiped off and are shiny. My father-in-law loves to squirt WD40 and engine starter in the plug hole (but misses sometimes). I checked the gap using my eye and the magnets and flywheel does not rub nor does it seem to be out-of-round. The magnets pass the 1/4" screwdriver pull test. The ignition module/magnet gap is about 2 sheets of paper thick (about 0.008 inches I believe). The resistance from the spark plug wire to ground is about 3,300 ohms. I've measured the cutoff switch and it is ok. Even tried disconnecting it. The ignition module protrusion is not centered on the magnets; it is to one side as the protrusion and magnet edges line up. The magnet width is about twice that of the protrusion. Is centering necessary (the weedeater has worked for 4 years)? Any ideas on getting this thing sparking correctly?
Doug

 
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08-19-13, 10:56 AM   #2  
Not sure how you can tell that the plug is not sparking when in the cylinder.
I'm thinking more of a gas problem. I would shoot a little starter fluid down the throat of the carb and if it then fires for a few seconds, I would be checking for fuel problems.


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08-19-13, 11:42 AM   #3  
On a two stroke, a good place to start is changing the plug. The blue spark is a lot better than the yellow. The latter is weaker.

A spark outside the combustion chamber isn't necessarily one that fires in a compressed air environment. If you change the plug without good results, you could well have an ignition module going out.

 
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08-19-13, 11:55 AM   #4  
Let me rephrase that...

Sorry about that: let me elaborate: the spark plug is never connected; it is either in the hole or not (compression or no compression). The plug wire is always connected to the spark tester. An aside: does using a nonresistor plug in lieu of the specified resistor plug matter? Does it interfere with the ignition module?

 
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08-19-13, 12:20 PM   #5  
Check the compression, low end weedeaters are terrible about scoring the cylinder walls and piston. Have a good one. Geo

 
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08-19-13, 12:45 PM   #6  
Having the plug wire connected to a tester you would get the same spark, plug in hole or not. On an engine that small you can easily turn the engine over fast enough to generate the same spark - plug in/or out.

 
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08-19-13, 12:52 PM   #7  
That's the problem...

The problem is I get spark with the plug out of the hole, no spark with plug in the hole. I can pull the starter rope as hard as I can, and I can pull very hard, and I get no spark if the plug is in the hole.

 
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08-19-13, 03:33 PM   #8  
The problem is the spark is having to jump two gaps. With the plug out of the hole you have two low pressure environments for the spark which allows the spark at both the plug and the tester. The ultimate ground for the spark is the ground electrode of the spark plug. I take it that is grounded to the engine head.

The positive current is strong enough to handle the two low presssures. But when you put the compressed air in the second gap the spark can't make it across. Since you've interrupted the positive current path before it hits ground, the tester gap doesn't have a path to ground = no spark when the spark plug is in the hole.

The issue of the compressed air environment is why a weak spark outside the head doesn't work inside the head.

 
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08-19-13, 04:04 PM   #9  
Thanks for your repeated responses.
The spark plug has nothing attached to it; I use it to enable/disable compression. The spark tester is from the spark plug wire through the tester to metal ground (the tester has an alligator clip on it which is clipped to the cooling fins of the engine). I test this in a dark garage.
I think, even though I pull the starter rope hard, the flywheel is not spinning fast enough when there is compression (plug is in the hole) though it sounds like it is spinning fast.
Does the ignition armature pickup have to be centered over the magnets? It currently is not, though the magnets encompass the entire pickup when spinning by.

 
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08-19-13, 04:40 PM   #10  
No the pickup doesn't have to be centered. The magnets present a magnetic field to the pickup and that field energizes the capacitor and coil. The capacitor loads then unloads with the passing of the field. When it unloads the coil fires. The closer you get to the source of the field, the stronger it gets. As long as the magnets pass under the pickups, you're good.

If you're seeing a difference in the spark in the tester setup as you've described, the difference in the spark/no spark isn't that much of a difference in the speed you're spinning the engine over, but rather the module is weak and needing replaced.

To put it in the context you're looking at, you may be spinning it a little slower, but if the coil was good, it wouldn't make any difference.

I take it you're having trouble starting the engine? Weak ignition module = bad coil.

 
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08-19-13, 08:58 PM   #11  
Sounds like marbobj has nailed it down.


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09-23-13, 11:37 AM   #12  
case solved?

Since I was so focused on the spark issue, I overlooked other contributing factors. I measured the compression and it turned out to be around 40 psi...terrible. I was informed by a small engines tech that it should be around 100 psi.
I think that a batch of gasoline, light on the oil, was used.
You were right, geogrubb.

 
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