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Garden Tractors and Mowers

Briggs & Stratton Ignition failure (no spark)

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Old 06-02-2011, 11:50:55 AM
jools jools is offline
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Default Briggs & Stratton Ignition failure (no spark)

Have a B&S lawnmower with a 3 HP magneto and points engine (model 9 I think).

Was running ok (bit difficult to start) – then just stopped – now no spark at the plug & won’t start.
Tried new plug – still no spark.
Condenser and points all fine & clean no rust or oil etc.
Got confused as when I checked DC resistance between condenser point connection and ground (with everything connected & points open) I got the continuity buzz – so assumed something was shorting to ground from the condenser point. Suspected the kill switch so disconnected kill switch wire to check it - but it was ok i.e. infinite resistance unless kill switch “made” when it was around 0.4 ohm.
Checked condenser with continuity meter – it has not gone short to ground (infinite DC resistance) – haven’t checked other properties - (does a faulty condenser stop these models from running altogether ?)
Then removed and checked magneto coil - Resistance of the primary = 0.5 ohm between wire to condenser point and ground wire / secondary resistance between ground wire and spark plug connection = 3,000 ohms.
So I assume the “shorting to ground” I saw at the condenser point was actually the resistance of the primary coil (0.5 ohm) - I would have expected this resistance to be higher than the kill switch “made” resistance (rather than almost the same value) but I’m no expert.

Have contacted B&S tech help line with some queries including asking what the magneto coil resistances should be – they said “Sorry but there are no published resistance readings for our ignition coils” – well thanks a bunch – no help there then !

Tried old condenser (removed from same mower some time ago) still no spark
Removed kill wire from condenser point connection (just to check kill wire again) – still no spark.

Also when I checked the flywheel magnets I was confused – one pole was ok (would attract screwdriver etc but not that strongly) but the opposite pole has no magnetism – is this correct ?

Should I recharge the magnets as a matter of course if so does it matter which polarity is used ?

So I assume I’ve either missed something obvious or in order of cost/ease it’s:

1. Re-magnetise flywheel magnets
2. Get a new condenser
3. Get a new magneto coil (I assume that even tho’ the coil resistance measurements look ok the coil is breaking down in some way when actually asked to produce a spark). If it is the coil I guess I should convert to a magnetron system (is the system ok ?) rather than try to find a second hand magneto coil like I have ?

Grateful of any help/comments from anyone ?

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Old 06-02-2011, 11:59:39 AM
wheelhorseman wheelhorseman is offline
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Default Re: Briggs & Stratton Ignition failure (no spark)

Best advice is to ditch the old points setup and go get a Magnetron electronic coil from Briggs. They are fairly reasonable..probably 30.00 fr the little 3hp. motor and will save you a lot of headache.
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Old 06-02-2011, 03:03:03 PM
K D Redd K D Redd is offline
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Default Re: Briggs & Stratton Ignition failure (no spark)

If this engine is for sure a Model 9, YOU CAN NOT use the Briggs Magnetron or any type of electronic replacement for the points and condenser as THIS ENGINE USED THE BRIGGS MAGNEMATIC magneto. The Magnematic mag. has a mechanical ignition advance system built into the camshaft. If a solid-state ignition is used with this mag, when you try to start it the ignition will be so far advance the engine WILL jerk the starter rope from your hand when you try to start it. You can not use the Magnetron system BECAUSE the magneto magnets ARE NOT in the flywheel. The magnets are in a separate hub on the crankshaft under the flywheel. Also the flywheel IS NOT keyed to the crankshaft.
A Mag Coil primary( The wire connected to the points) has a VERY LOW resistance. It is about 1/2 OHM with the engine block as the common point. It is the rare ohmmeter that can tell the difference between the resistance of the coils primary and the ZERO resistance of a closed set of points. You need to also read the resistance of the spark plug wire to engine ground. It should be in the Kohm range. I have seen as low as 4 Kohm to as high as 13 Kohms. No reading is normally BAD but not always. I was checking all of my old Briggs engines for a show. All had fire but two. On one the the plug wire had came loose from the coil, easy fix. The other I thought the coil had went bad because my Ohmmeter show NO reading/open on the plug wire. I also notice the points had oil on them. when I cleaned the point I had fire even though the high tension side read open. At best using a ohmmeter to check a mag coil is better than nothing test BUT there are ISSUES. THE ONLY TRUE TEST OF A MAG COIL IS ON A MAG COIL TESTER.
I would still bet you coil is bad because the MAGNEMATIC coil had issues. I COLLECT the number series Briggs engine so I know the Magnematic mag well. The coil, Briggs P/N 290880, is No Longer Available from Briggs. When it was the cost was about $100.00. This was so high, I designed a way to use a coil from a later Briggs on the Magnematic magneto. Recently I have found that Tulsa Engine Warehouse ( ) has a made in China replacement for the Briggs 290880, their P/N 29-1030 ( cost about $21.00). I DO NOT know it's quality as I have never used one of these coils.

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Old 06-03-2011, 07:04:37 AM
jools jools is offline
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Default Re: Briggs & Stratton Ignition failure (no spark)

Thanks guys for our thoughts - my mistake it's not a model 9 as it has the magnets in the flywheel and no mechanical advance system (I think the guy who sold it me said it was a model 92902 which I wrongly assumed was a model 9).
Have since checked both capacitors that I tried & they measured out at 0.22 micro Farad so I think they're ok.
So I assume it is the coil breaking down under load & will try an electronic coil.
Any thoughts on whether I should re-magnetise the flywheel magnet ?
I thought the flywheel had two magnets opposite each other but on closer inspection I think it just has one magnet (comprising three steel inserts S-N-S config'n).
The opposite side (which has two steel inserts) I guess is just a counter weight and therefore does not need to be magnetic ?


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