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Old Fix it Guy,Paramedic
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Discussion Starter · #1 ·
I Have a Sears mower about a 1993. A 12.5 B/S engine single cylinder. I had mice build a nest and chew the spark plug wire a few years ago. I taped it and it ran good. Then engine started taking 20 tries to start. Replaced the magneto. Still takes 20 tries to start.Instant start is no help. The oldest son mowed an acre it stopped. Now No Start. My question is,you need + power to the magneto with the key on correct? Have no + to the lead.Does the mower neck have a switch? I have bypassed the dead man switch on the seat,it was bad. The wire from the ignition switch to the magneto does have continuity so it's intact. I replaced the ignition switch today also.
 

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" My question is,you need + power to the magneto with the key on correct? "

ABSOLUTELY NOT. The ignition is completely separate and self contained. Any outside current to it will destroy it. There will be a safety switch that prevents the engine from starting when the mower deck is engaged. You don't say if it is manually engaged or has an electric clutch but either way, it will have a safety switch. You did disengage the deck after it stopped and before trying to start?

As for difficulty starting, check that the choke is actually completely closing when the throttle cable is in the choke position. Adjust cable if necessary.

Walt Conner
 

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" My question is,you need + power to the magneto with the key on correct? "

ABSOLUTELY NOT. The ignition is completely separate and self contained. Any outside current to it will destroy it. There will be a safety switch that prevents the engine from starting when the mower deck is engaged. You don't say if it is manually engaged or has an electric clutch but either way, it will have a safety switch. You did disengage the deck after it stopped and before trying to start?

As for difficulty starting, check that the choke is actually completely closing when the throttle cable is in the choke position. Adjust cable if necessary.

Walt Conner
:ditto::ditto::ditto:
 

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Old Fix it Guy,Paramedic
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Discussion Starter · #4 ·
It's a manual mower engage. The starter will not engage when the mower is in gear. It seems to have a weak if not any spark. It's a no start even with doses of Instant Start. I take it the single wire coming off is for charging the battery? So what potential is it + Or -? How does turning off the key kill the engine? The choke does engage, I cleaned off all the gunk on all the carb and linkages.It's got a new spark plug,magneto,air cleaner,belts,fuel & air filters and oil is changed every month.
 

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Turning the key off grounds the magneto, so the spark dies. Putting voltage on that lead will damage the magneto. You should have a wire (probably black) that comes out from the mag, and another wire (maybe red) that eventually goes to the battery to charge it.
If you disconnect the mag grounding wire, you should have a fat blue spark at the plug, which you will ground to the engine while testing. You could try starting it with the mag grounding wire disconnected. If it does start, the key will not kill the engine, (you have it disconnected) you will have to do it another way.
If you do have spark, does the plug get wet with gas while cranking? If no, you may not be getting fuel into the engine.
Another common problem is that the flywheel key shears, throwing the timing off, and on some engines causing a no-spark condition. To check it, you'll need to remove the flywheel nut, and maybe the flywheel itself. Even if the key is only partially sheared, it will affect your spark.
20 pulls to start indicates to me a fuel problem. Have you tried it with the tank full to the top? Some carbs have trouble pulling fuel up from a tank that's low on gas.
 

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Old Fix it Guy,Paramedic
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Discussion Starter · #6 ·
Thanks for the infomation. I have never had to get this deep into fixing a piece of small equipment. I have kept 2 snow plow trucks on the road and working for many years. I am far from a mechanical rookie. This is just a new animal for me. But I am a fast learner. Dad used to say the more you learn the more you know! I think I will look for the black wire to be going to ground somewhere. I read the sticky on the carb this engine has. So I will look at it tomorrow too. The real kicker is that it does nothing even with Instant Start sprayed into the air cleaner housing. That's why I was thinking it's ignition problem. I NEVER have simple problems.
 

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The black wire is grounded through the ignition switch. When you shut the machine off, the switch grounds the mag. Don't expect the wire to go directly to ground - if it did, you'd never get spark. When you turn the ignition on, you merely un-ground the mag wire.
 

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Old Fix it Guy,Paramedic
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Discussion Starter · #8 ·
It has another black lead connected to the mag line that must be going to a switch on mower etc. I do have a ground with the black wire connected and key on.
I took the black wire off the mag, no joy, no start! I think I will look closer at the carb. May not be getting any air flow. I am starting to lean toward a sheared timing pin. Makes sense, no start with instant start not even a sputter. My Son's run over everything! They stop the engine 4 times a mow running over stuff and ant hills. The property has 200+ feet of frontage on a main road to the National Park.
 

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Forget the carb for now, you need to get spark. Check the flywheel key. With all of the sudden stops, the smart money will be on that! I have $0.02 to bet....
 

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I agree,check the key in the flywheel.There is also a fuse for the ignition,actually 2 on some.I had one burn out and no way it would go.But it does sound like the flywheel key.
 

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I agree,check the key in the flywheel.There is also a fuse for the ignition,actually 2 on some.I had one burn out and no way it would go.But it does sound like the flywheel key.
It will still spark with the flywheel key screwed up. It just won't spark at the proper time.

To the original poster, you mentioned that you replaced the magneto. Did you make sure you didn't put it on backwards (the magneto should have "cyl side" stamped on one side of the black plastic, and assuming this is a vertical shaft engine you shouldn't be able to read it from the top). Secondly, did you get the clearance between the flywheel and the magneto correct? When you place the card between the flywheel and coil make sure you have the flywheel rotated to where the magnet is. Even the slightest amount of extra clearance will give a weaker spark.
 

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It will still spark with the flywheel key screwed up. It just won't spark at the proper time.

To the original poster, you mentioned that you replaced the magneto. Did you make sure you didn't put it on backwards (the magneto should have "cyl side" stamped on one side of the black plastic, and assuming this is a vertical shaft engine you shouldn't be able to read it from the top). Secondly, did you get the clearance between the flywheel and the magneto correct? When you place the card between the flywheel and coil make sure you have the flywheel rotated to where the magnet is. Even the slightest amount of extra clearance will give a weaker spark.
and you did not connect the magneto black lead to 12V
 

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Old Fix it Guy,Paramedic
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Discussion Starter · #13 ·
I got a new key and pulled the fly wheel. The key was fine,put the new one in anyway. I got it back together and it still does not fire up. Then the battery decides to die. The tractor is 5 miles from home and I have no electric there to charge the battery. I looked up the air gap for the ignition. I will check it tomorrow. No hot lead hooked up to the ignition. Took the black wire loose still no start. It mowed the 3 acres 2 times with the new ignition on it. Before it died on the 3rd mow. I am thinking the carb has died!
 
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