coil problems

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  #1  
Old 06-26-08, 11:12 PM
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coil problems

I have a riding mower with a 18.5 B&S engine
model 42a707 type 2238 E1 code 98012158

After using my own small knowledge of small engines and reading the forums i determined that my coil was bad. Reasons being is that the spark while cold seemed weak and white in color and after the engine ran for 30 mins to an hour it would shut off and I wouldn't have any fire at the plug. So I bought a new coil at my local shop sanded the flywheel and installed it. Problem is it seems to be doing the exact same thing. just so you know the new coil is made by Oregon "part # 33-345". After reading the forums im thinking i should have tried to find a B&S coil instead but its to late for that. So first off is there any better way for me to check the coil than trying to watch the spark at the plug? And what should the gap be at the coil to the flywheel?
 
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  #2  
Old 06-26-08, 11:34 PM
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The gap should be .010". I haven't had luck with aftermarket coils and I don't use them at all. Checking spark is the best way to determine if the coil is good or not. If there is sufficient spark, it's good. If there is no spark, disconnect the kill wire and check again. If still no spark, the coil is bad.
 
  #3  
Old 06-26-08, 11:42 PM
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Originally Posted by cheese View Post
The gap should be .010". I haven't had luck with aftermarket coils and I don't use them at all. Checking spark is the best way to determine if the coil is good or not. If there is sufficient spark, it's good. If there is no spark, disconnect the kill wire and check again. If still no spark, the coil is bad.
I was pretty sure that would be the answer id get but wanted to check just to make sure. Ahwell its 1:40 am here so the neighbors might get upset with me if i play with it now guess it will have to wait till morning. Thanks for your help.
 
  #4  
Old 06-27-08, 09:00 AM
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There should also be a capacitor or semiconductor device to quickly quench the coil mag field and give you a much better spark. It's in there somewhere. In the old days it was across the breaker points.
 
  #5  
Old 06-27-08, 10:27 AM
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Couple of comments... I would invest in a 'gap type' spark tester. Good news is they are inexpensive and available at most auto parts stores. Set the gap for a quarter inch or so and you can leave it inline while the engine is running. If you are fortunate enough to be watching it when the engine dies, it will be fairly obvious if the spark going away is the source of the engine failure. If not, when you try and turn it over, you'll see if you have spark or not.

IMHO, and that of many other seasoned small engine repair folks, the idea of a 'weak spark' is an extremely rare occurrence. You either have it or you don't. In my 30+ years of servicing small engines, I've never run into a weak spark(assuming the magneto gap is anywhere close to spec). That's why the gap type tester is so valuable. If you have spark there, it tells you the other ignition components and kill switch wiring is fine.

Keep us posted!
 
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Old 06-27-08, 10:16 PM
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There's no capacitor or semiconductors to worry with...the entire ignition system consists of the flywheel and coil/armature assembly.

I agree that weak spark is very uncommon on todays engines (used to see it a lot on older breaker-point ignition systems, especially when the capacitors got leaky). I do see it occasionally, mostly on a few japanese models and the opposed twin cylinder briggs engines like yours. It's not common though.
 
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Old 06-28-08, 07:15 AM
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the entire ignition system

How do they pulse the coil and get the spark from rapidly collapsing the mag. field?

The last mower schematic I looked at had some kind of snap-action semiconductor device that replaced the points and condensor. The magnets flying past the coil give more of a sine wave than a square wave, and I thought you still need a square wave. I guess this thing could be integrated into the coil.

?
 
  #8  
Old 06-28-08, 04:03 PM
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The passing magnet induces a current into the windings in the coil which is then collapsed by the triggering of a transistor which grounds the windings at the appropriate time (determined by the position of the magnet). These components are contained within the coil assembly and are not visible or removeable. On briggs engines, the semiconductors and pickup coil used to be contained in a snap-on module like you described but that hasn't been done since the '80s, maybe early '90s. Today, the components are a unit and non-serviceable.

I'm not sure the waveform present matters because it is the passing magnetic field that is needed, and generally the waveforms matter when talking about electrical current, not magnetic fields. The waveform comes more into play when talking about the charging system and related magnets. There, you start with an electrical sine wave which is converted simply with a diode.
 
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Old 06-28-08, 05:04 PM
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The passing magnet

Thanks.
I have at least one lawnmower waiting to fail and this info may save me a trip to Border's.
 
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