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Briggs & Stratton no spark

This is a discussion on Briggs & Stratton no spark within the Small Engines forums, part of the Tech Support Forum category. My Brigggs & Stratton lawn mower engine (model 127802) has been running good. After running for 15 minutes one day


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Old 10-12-2008, 10:23 AM   #1
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My Brigggs & Stratton lawn mower engine (model 127802) has been running good. After running for 15 minutes one day I shut it down. Five minutes later it would not start. Checking I found no spark (I can hold the spark plug wire during the test, no shock). I removed the kill wire from the magneto armature, no improvement. Gap to the flywheel is OK. Resistance to ground for the spark wire lead is 4500 ohms. Checked for spark with and without a spark plug. Spark is adequately clean and dry (no oil). Tried a replacement armature, no improvement. The magnets in the flywheel appear strong but I have no way to test them. The flywheel is correctly aligned with the vertical shaft, key is OK, but I don't believe a problem there should affect spark. What am I missing? What else is there that could affect the spark?

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Old 10-12-2008, 01:10 PM   #2
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Hi & welcome to TSF
Given you have checked timing and clearances - sounds like the "firing unit" or plug lead connection lead to me.

Have you replaced the transistor unit - as they are more likely to fail than a coil, and short of setting the engine on fire - never heard of flywheel magnets failing over a short period of time.

Also, make sure the connection between the coil and the spark plug lead is good - they often just screw into the coil connection and can turn sour - even better - look at replacing the spark plug lead - as they can give up

Let us all know how you go

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Old 10-13-2008, 08:01 AM   #3
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Thanks for the quick reply. My engine does not have a transistor unit as far as I can see. The total electronics seem to be the coil and the magnets in the flywheel. The coil, on a laminated iron core, has two wires. One is the kill wire. The second is the spark plug wire which is molded into the blob of plastic containing the coil. There must be a need for a ground also through the core to the engine casting. Since I can measure a steady 4500 ohms from the spark plug wire to the engine casting the ground connection must be good. There doesn't seem to be a mechanism, like points, to open and make a spark.???
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Old 10-13-2008, 11:12 AM   #4
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The B&S coils are call Magnetron and use no external parts to work. It can go on back wards, so make sure you can read "this side out" not "cylinder side" cast into the coil lamination plastic casing. If you have spark it's probably a fuel issue, try a shot of starting fluid. If your not getting spark; are you installing the rewind and using the rope to pull it over? The Magnetron coils needs more RPM's than a point style magneto and you can't test a Magnetron coil by spinning the flywheel over by hand. Always perform tests with the shorting wire in the back of the coil disconnected.
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Old 10-16-2008, 05:17 PM   #5
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Thanks for the reply. I'm sure that the unit is not on backward, on the mower model I have one side of the core is shaped to fit the curvature of the flywheel so there is only one way to install it. I was spinning the flywheel to check for a spark but with the mower fully reassembled, except for the disconnected shorting wire, and using the rope pull there was still no spark. I'm still thinking about this problem, I have not yet given up.
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Old 10-16-2008, 05:40 PM   #6
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This is bizarre The magnets in the flywheel rarely if ever fail - so if they will hold a steel nail with reasonable strength -we can rule that out.

So all I can think of is - do you have a good connection between the chassis of the coil unit and the engine block?? - ie - give it a bit of a sand with sand paper so that there are clean metal contacts between the block and the coil unit - just to ensure you are getting a good circuit thru the magneto.

Lastly - is it possible that the new coil unit was a dud (ie DoA)??? - Maybe you can see if the shop that sold it to you would do a replacement / exchange - coz provided you have the earth lead (kill wire) disconnected and the coil / firing mechanism is good - you just have to get a spark. If you pull the engine thru - can you feel any voltage in your hand if you hold the plug lead???
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Old 10-17-2008, 08:09 AM   #7
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Quote:
Originally Posted by charles6903 View Post
Thanks for the reply. I'm sure that the unit is not on backward, on the mower model I have one side of the core is shaped to fit the curvature of the flywheel so there is only one way to install it. I was spinning the flywheel to check for a spark but with the mower fully reassembled, except for the disconnected shorting wire, and using the rope pull there was still no spark. I'm still thinking about this problem, I have not yet given up.
Can you read "this side out" printed/cast into the plastic on the coil? The wrong side out will say "cylinder side". They can be put on upside down, of course the curved side has to go toward the flywheel, but you can flip it over and it will also fit. If you got an original B&S coil the probability of it being bad are very low.
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Old 10-18-2008, 12:33 PM   #8
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You did it! The coil was mounted upside down. When turned over the engine ran without any problem. Very embaressing to say the least. My only excuse is that I mounted the replacement coil that I had bought so that the grounding tab on the coil was pointed toward the grounding wire. When the coil is mounted correctly the grounding wire has to wrap around the coil to reach the grounding tab, and it must be connected before mounting the coil since when the coil is mounted the grounding tab is hidden.

So, the bottom line is : no spark, tested coil without moving the coil, checked out other suggestions, still no spark, bought and replaced the coil, still no spark, turned coil over (to proper orientation) and restored operation.

Thanks very much to all you guys for hanging with me. Chuck
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Old 10-18-2008, 12:40 PM   #9
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Not a problem and happy to help - and glad you have it all sorted - now, looks like it's back to mowing
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Old 10-18-2008, 02:06 PM   #10
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Your welcome. It's actually quite a common error, I wish they would use white paint, to make it easier to read.

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