Small Engine Riding Lawnmower

[Chief Brody]

I got my riding lawnmower out today...Briggs and Stratton 12HP

Won't start...nearly burned the starter up trying to start it...I don't seem to be getting any fire except every now and then...

I took the cover off and cleaned all the rust off the flywheel and that Magneto thing....still nothing...

What could it be?

 

[snowdriver]

fuel??

 

[Chief Brody]

fuel??

not fuel...plenty in carb...

 

[Chief Brody]

I pull the spark plug cable off and hold it near plug and crank it and no spark

 

[diesel4me]

If its newer than 1982,a B&S wont have points and a condensor ,it should have a coil with a sensor that senses when the flywheel magnet passes by it and tells the coil to fire..usually the coils dont die that often,and most often when they do,it'll will start and run till it reaches operating temparature,then quit..

It could be the ignition switch is grounding out the coil--but I'd also make sure all the "safety" switches are not killing the spark ,there is usually several on a tractor--one on the seat to kill the engine if you dont sit on it,another on the lever that turns on the blades,and there might be one on the shifter so it wont start unless its in neutral..

The quickest way to see if one of those switches is killing the spark,after making sure the blades are shut "off" ,(its easy to forget that!) -is to find the "kill" wire that connects to the coil ,usually a black one,that comes from the chassis or wiring harness on the tractor..by unplugging the wire at the engine,it disables the safety switches...on most B&S engines the connection for that wire is near the throttle cable bracket,it has a 3/8" nut on it,the wire will usually have a slip on spade connector or a ring ,so you may need to take the nut off--leave any other wires coming out of the engine connected,only remove the one going to the chassis or harness..
try taking that wire off and see if it has spark then..

 

[Chief Brody]

If its newer than 1982,a B&S wont have points and a condensor ,it should have a coil with a sensor that senses when the flywheel magnet passes by it and tells the coil to fire.

yep, that's what I called a "magneto"

It could be the ignition switch is grounding out the coil--but I'd also make sure all the "safety" switches are not killing the spark ,there is usually several on a tractor

I know it's insane, but I removed them all 2 years ago....even on the transmission

The quickest way to see if one of those switches is killing the spark,after making sure the blades are shut "off" ,(its easy to forget that!) -is to find the "kill" wire that connects to the coil ,usually a black one,that comes from the chassis or wiring harness on the tractor..by unplugging the wire at the engine,it disables the safety switches...on most B&S engines the connection for that wire is near the throttle cable bracket,it has a 3/8" nut on it,the wire will usually have a slip on spade connector or a ring ,so you may need to take the nut off--leave any other wires coming out of the engine connected,only remove the one going to the chassis or harness..
try taking that wire off and see if it has spark then..

I found two wires there...one coming form the coil and one is literally in the the throttle cable...

 

[Chief Brody]

So I need to remove the wire from that bracket that is coming from the coil? It is the one that connects by a spade connecter at the coil end?

 

[79HeavyChevy]

You did set the magneto gap correctly right? The way I was always taught was to clean it with a brillo pad or sand paper then take a dollar bill and fold it in half and put it between the magneto and the flywheel to set the gap.

 

[Chief Brody]

You did set the magneto gap correctly right? The way I was always taught was to clean it with a brillo pad or sand paper then take a dollar bill and fold it in half and put it between the magneto and the flywheel to set the gap.

even if I didn't, wouldn't there be a spark...doesn't gap just advance or retard?

 

[79HeavyChevy]

I don't really know man, I just know I've set many like that and they ran.

 

[Chief Brody]

it makes more sense to me that something is killing the coil...that's probably why it will sputter just a little...I can't wait to get out there tomorrow and remove that ground wire from the coil...I bet that solves it

 

[diesel4me]

Contrary to popular belief,rust on the flywheel magnet and coil laminations doesn't really affect its ability to produce a spark...as long as the magnet has not lost its magnetism it should still create enough current to fire a plug OK..the air gap between the 2 parts needs to be close as possible without rubbing,most specs range from 5 to 10 thousandths,I just use a peice of plastic from something like a bleach bottle,just put it between the coil and flywheel magnet and loosen the bolts on the coil,and the magnet will "clamp" it together,and then re-tighten the bolts..I do sand rust off,but only to please my desire to make things look better,not so much "do" anything for it..

The black coil primary "kill wire" coming out of the engine should attach to a stud on the throttle control,thats where they would join the rest of the safety switch wires--you will want to unplug the wire coming out of the engine (make sure its not one of the two or more wires that come from the stator to charge the battery or run the headlamps)...

If you still get no spark with that wire off and not touching ground,it could be the coil died during storage..sometimes moisture will build up in a coil and short the windings out..some guys claim "baking" one at low temparature in an oven will bring one back to life,but its only worked for me a few times and was temporary...a new coil will run about 20-30 bucks probably..bring the old one as there may be several that fit that engine,and dont be tempted to swap one on that doesn't look the same from a push mower,they are different ,even if they will bolt up..

You may want to try a new spark plug,sometimes a fresh one has less resistance and a weak magneto will fire one,when a used one will not let it..all it takes is a crack in the porcelain or the plug to foul and it will want to fire up inside the shell instead of at the two electrodes..magnetos are fussy,they dont have the power of a 12V coil to fire a fouled or cracked plug..

If your not 100% sure its lack of spark keeping it from running,it could be a valve has stuck from sitting ,or the carb or gas tank got water from condensation in it and the mixture is too weak to fire properly..I squirt a little gas in the plug hole rather than use starting fluid,that stuff can blow the head gasket or do internal damage to the aluminum connecting rod if your not careful with it..also washes the cylinder dry and can make it scuff up too..

 

[Chief Brody]

You nailed it....when I removed the black wire from the coil it started right up...already mowed the lawn....now got to figure out a mechanism to turn it off...right now I have to ground the wire.

I wired in the safety switch on the seat, but it is reversed....when you sit on the seat it makes a connection...I need one that breaks the connection when you sit on the seat and makes the connection when you stand off

 

[diesel4me]

Glad you "fixed" it without needing parts or $$$$ thrown at it!...(I'm never that lucky!)...you can use a simple toggle or rocker switch to shut the engine off,just hook that black wire to one side of the switch and ground the other side..
On push mowers I have used the good old "kill strap" that you press against the spark plug boot,a strip of tin bolted to a head bolt is all you need..

I've even used a house light switch and cover plate on one of my "beater" tractors I use as a yard tug and snowplow to kill the engine..

I also wired up a old fashioned starter button (looks like a dimmer switch) to it when the ignition switch fried,it beat paying 30 bucks for the special ignition switch it takes--it has a starter/generator and they designed the ignition switch with heavy copper lugs so no starter solenoid was needed--but they often fail from handling all the amps,a rather piss poor design..the starter push button was 10 bucks at NAPA and the house switch and cover I had already..
I suppose I could have added a ford starter relay and a push button instead,but the switches were cheap, and less hassle to install..

 

[Chief Brody]

I'll buy a little on-off push button or toggle and do it that way....drill a hole next to the keyed ignition...make it look like it belongs there.

I've been dealing with this issue for two years now...that's why I clipped all the safety switches out of there because I couldn't locate the problem...I would try to start it till it began to flood and occasionally it would actually start and run long enough for me to finish the lawn...

the intermittent thing is what threw me for a loop.