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xenon172

A no spark puzzler

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xenon172
Had one of those head scratchers today with a Briggs 5 hp that had no spark. It is a 136212 horizontal shaft L head with the most basic ignition system possible, a pull start and a standard kill switch. It is a newer motor, 1995, so there are no points or condenser to make things complicated, just a standard electronic module. First thing the kill switch was eliminated as the most likely problem. There was no unusual play in the flywheel and the magnet seamed OK firmly holding a large socket. Reset the air gap and still nothing which left the module. Picked up a new module and installed it and still no spark. This is where the head scratching begins because there's nothing left since even a bad flywheel key wouldn't interfere with spark. The only component that hadn't been changed out was the flywheel and we had one that was salvaged from a blown engine so we put that in. While setting the air gap there was no question that the magnet in this flywheel was noticeably stronger. First pull and there was a good strong spark. Even though the old flywheel magnet seemed strong it was obviously not strong enough and I don't know of any practical way to check magnet strength. It was an interesting problem that I have not seen before.

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larry8200
I once read something like "Take a large screwdriver and place the tip on the magnet, if it will hold it there with the handle sticking straight out the magnet is OK". Not very scientific and a little ambiguous. But even if you had some expensive lab equipment to measure magnetic fields, being able to get the values for all the different makes and models seems pretty unlikely. I'm interested to see what get's posted here.

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MrSteele
I have seen the problem before, but rarely. An old trick my father used when storing engines was to be certain that the magnet was located at the armature, so the magnet would be drawing on something. Not certain if it worked, as all those engines were stolen and sold for scrap, along with the cast iron Clintons and Briggs I had been amassing for the last few years. Occasionally, I get an engine that has weak spark, and by decreasing the air gap, increase the spark. You have to set it so that the flywheel will not strike the mag, and leave some room for expansion. It might not be the best solution, but the ZZ on the walking tractor has been running for years adjusted thusly. I know that the gap is enough for a hot engine, as I use the tractor for breaking ground, it runs hot in that process.

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xenon172
The old flywheel magnet would have easily passed the screwdriver test which is why we figured it was good. Also set the air gap about .002" below the minimum and that wasn't enough but as far as I was willing to go. I have run into bad magnets before but never one that was this deceptive being only half bad

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