Testing a lawnmower Magneto

Hey all,

So...the POS...I mean Poulan Pro push mower with a Briggs and Stratton "550 Series" engine on it quit running well last year. It'll start up and run for...oh...30 seconds, give or take, and then just shuts off like I hit the kill switch or it ran out of gas. It usually won't start backup right away...but it always will eventually, and it'll do the same routine.

I automatically assumed it was the carburetor, waited 6 months (like you do), and then I cleaned the carburetor well (ultrasonic baby!) and put a new diaphragm and gasket in it. I cleaned and dried the fuel tank as well.

I have NOT put a new spark plug on it yet. The one on there was replaced last year trying to get it to run right...but I'll probably just get a new one becasue it's the cheapest part of the system.

Now I'm left wondering if it's maybe the Magneto that's gone bad (intermittently). I unplugged the kill switch to see if it was shorted. I put electrical tape on the spark plug wire in case it had shorted to ground where it comes through the cowling. All had the same results. Run for a while and die abruptly.

Can I test the mag with my Digital Multimeter? What am I looking for, if so?

Thanks!
Clem

I've been fiddling with lawnmowers since I was 12. So, like 23+ years now. I used to be able to pick up a mower from the 1980's or earlier, breathe on it a bit, and bring it back to life. The newer crap that's out there now is so dang frustrating, though. Nothing's adjustable, everything is throwaway.

I have a JD 265 tractor now...it runs for about an hour or so and then begins intermittently cutting out. I finally realized that when the fuel in the tank gets below about 1/2 the engine's getting starved. Either something wrong with the line, the tank pickup, or the cheezy PCV-powered fuel pump. For now I just make sure it's always got a full tank when I start using it.

Next time you run the machine, and it dies, immediately pull off the spark plug boot, stick a spare plug into it, hold the side electrode against something like the cylinder head fins (make sure the fins are good and clean and you wear thick gloves and/or you don't have a pacemaker) and crank the engine over. If you don't see a good, healthy spark, well, there you go.

I guess I forgot to mention one thing I did:

I had the girlfriend (and owner of this particular, non-running, mower) look for spark while I pulled the cord.

She saw spark one time. I put the plug in, fired it up until it died, pulled the plug to check for spark and...No spark.

But there are too many variables there...

I want a little bit more of a bench-test. Should I be looking at continuity or a particular resistance across some leads/poles on the magneto? Or...???

Thanks!
Clem

If it were me, at this point I'd either a) replace the magneto, or b) depending on the age/price of the mower, get a new mower. Check online for part pricing- it's MUCH cheaper than at a dealer, generally.

Plugs, plugs plugs, thats a textbook bad plug, firing outside but not under pressure, buy NGK if you can. If it turns out a bad mag assembly, Ive heard of people fixing them by cooking in the over at 200*f for a few hours and dropping it in a bucket of WD40. The failure is usually just cracked potting wicking up moisture.

$30 on amazon for a mag.

I'll put a plug in it first and see if that helps.

New mower? While that may end up happening...it won't be because I gave up. The Girlfriend will get impatient first ;)

If I can't fixt it, I don't need to own it.

Of course...that's why I don't own a lot of the things people own.

I'd put money on the coil. The plug is a cheap test and would not hurt. Do a spark test without the plug - stick a phillips in the plug boot.

Run it until it stops, then immediately do a test with the screwdriver instead of the plug. If you have no spark, then its the coil. If you DO have spark, then try the plug. If you don't get spark through the plug, then its the plug.

If you have good spark but it still won't run, it still could be the plug or the coil. Its a lot harder for that little magneto to make a spark under pressure.

A bad tank vent (fuel cap) can also mimic a bad coil.

zipty842 wrote: A bad tank vent (fuel cap) can also mimic a bad coil.

bingo, but it usually takes longer than 30 seconds.

the cap vent hole or mesh under the hole gets clogged w/ dirt, no vent won't let fuel feed to carb bowl, starves the carb for fuel

True, but I did see it happen quick occasionally when I was in the business. Its an easy check to remove the cap and eliminate the possibility. I was on the 2 stroke side of things at our shop, but I remember our coil tester couldn't do magnetos removed from the mower.

When the magneto goes, you usually get no spark. To help ensure bafflement by you, spinning by hand will usually produce no spark.

From your description of it will start, then die in a few seconds, and resist starting again until some time goes by, I still say it's fuel.

Ultrasonic cleaners are nifty. But, they don't work worth a darn on this yellow/green stuff that forms from old ethanol blend fuels. That requires mechanical cleaning, usually in the form of reeming.

foxtrapper wrote: Ultrasonic cleaners are nifty. But, they don't work worth a darn on this yellow/green stuff that forms from old ethanol blend fuels. That requires mechanical cleaning, usually in the form of reeming.

There's truth there. I love the carb cleaner gallon paint bucket things, but I still got through every passageway with something akin to pipe cleaner and mechanically clean it all out, and then blow it out with compressed air (and usually a repeat in the dunk bucket) before I call it good.

I usually have a few plugs laying around, and if one fails to spark try a few more before declaring it an ignition problem. But if it won't spark immediately after it shuts off, but then does spark with the same plug an hour or two later, it sounds like the mag.