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hi there

Lately my b&s 4hp mower has been dying after about 10 minutes running.
I am thinking that it is the ignition coil that is the problem. Ive changed spark plugs and
put a new diaphram in the carby also cleaned it out.

What i did notice when i went to reset my air gap on the coil, is that one of the magnets seems
fine and strong on the flywheel. But the other magnet on the opposite side is a lot weaker. Is this normal?
Could it be the problem?
I dont want to waste money on a new ignition coil because their very pricey down here in oz. in fact a second
hand one cost the same as a brand new one in the USA.

hope you guys can help

cheers Phill.

The weak magnet on the opposite from the strong magnet is just a counterweight for the flywheel magnet on the other side. Next time it dies, remove the plug, insert it into the spark plug boot, hold plug again metal part of engine to ground it, and have someone crank the engine to check for fire. If no fire replace the ignition module. if still firing then need to check for other issues.

ILENGINE said:

The weak magnet on the opposite from the strong magnet is just a counterweight for the flywheel magnet on the other side. Next time it dies, remove the plug, insert it into the spark plug boot, hold plug again metal part of engine to ground it, and have someone crank the engine to check for fire. If no fire replace the ignition module. if still firing then need to check for other issues.

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Thanks for your reply! I didnt know about the counter weight.
There is a spark after it dies. Is it possible that when the coil gets hot it makes the spark weaker? Or do they always fail completely
when hot and have no spark at all?
I cant understand why it can run for 5 or 10 minutes and then die and not be able start again.

phil, The spark generated and visible on a plug held to ground may not be sufficient to fire reliably in operation. A you tube lawn mower mechanic dude showed how to make a poor man's tester. Take an old plug and cut off the electrode at the 90 degree bend area so the spark must jump 1/8." I had a trimmer with a weak pick up coil which would show spark ,but not run. It could not jump 1/8," but a new trimmer spark could. :thumbsup:

motoman said:

phil, The spark generated and visible on a plug held to ground may not be sufficient to fire reliably in operation. A you tube lawn mower mechanic dude showed how to make a poor man's tester. Take an old plug and cut off the electrode at the 90 degree bend area so the spark must jump 1/8." I had a trimmer with a weak pick up coil which would show spark ,but not run. It could not jump 1/8," but a new trimmer spark could. :thumbsup:

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yes ive seen that video. i did make the gap on an old plug about three times normal width and it had spark.
but i did not test it with that plug when the mower was hot.
I will try that tomorrow. its after midnight here and the neighbors might not appreciate it now lol
thanks for the reply!

phill said:

Thanks for your reply! I didnt know about the counter weight.
There is a spark after it dies. Is it possible that when the coil gets hot it makes the spark weaker? Or do they always fail completely
when hot and have no spark at all?
I cant understand why it can run for 5 or 10 minutes and then die and not be able start again.

Click to expand...

phil, although I have not cut one open I read that some coils contain electronic components...a capacitor and possible transistor. If so these things are notorious for failure at temp if bad. Just a thought.

motoman said:

phil, although I have not cut one open I read that some coils contain electronic components...a capacitor and possible transistor. If so these things are notorious for failure at temp if bad. Just a thought.

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motoman, I had no idea the coils had that stuff in there. Thanks

Today I bought a second hand coil put it on and check for spark first. It has a strong spark now.
Then i screwed in the plug and it started first go! I let it run untill it emptied the tank.
Now it purrs like a kitten.
Thanks for the help guys!