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What exactly does the fuel shutoff solenoid do on the bottom of the carb? A neighbor has a JD L105 lawn tractor. All of a sudden it started shutting off and surging real bad whenever it gets hot. They read somewhere that the fuel shutoff solenoid can cause this. They were going to simply unplug it but apparently it is in series with something because when unplugged the engine won't start.

So to disable it they took it out, cut off the plunger and reinstalled it. They felt comfortable with this drastic action because you can get a replacement for $8.00 on Ebay. The good news is this seemed to cure the shutting off and surging problem BUT... now the engine won't shut off at all. They had to choke it to stop the engine.

Now I always thought those little solenoids were to stop back firing by cutting off the fuel the moment you turned off the key. But are they the only thing that stops the engine? Wouldn't turning the key to off also remove power from the ignition circuit?

They've already ordered a replacement and it should be here today but I am curious why the engine would not shut off with the solenoid disabled.
 
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I would guess it is tied to a rollover switch...
 

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It is to shut off the fuel when you turn off the engine. It keeps extra fuel from getting into the cylinder, and therefore keeps it from backfiring.
 

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Its a normally closed solenoid/plunger, thats why unplugging it doesnt work. When you turn the key on it applies power to it to pull in the plunger.
 

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Their is some other delay circuitry involved to run the engine to use the fuel between the solenoid and the jets before power is removed from ignition. The solenoid is between the fuel bowl and the jets passages.

Reasons for fuel solenoid:
1.) Backfire prevention
2.) Air pollution from evaporation of an open fuel system.
 

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Discussion Starter #6
Thank you for confirming my understanding of the purpose of the solenoid. I'm still not understanding why the engine won't shut off when the plunger is removed from the solenoid.
 

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Disconnected switch grounds OR a disconnected magneto "trigger" wire can cause it not to shut off.
 

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Thank you for confirming my understanding of the purpose of the solenoid. I'm still not understanding why the engine won't shut off when the plunger is removed from the solenoid.
It' won't shut off because the plunger is cut off. That solenoid is the only thing that shuts down the engine. When you turn the key on the plunger retracts allowing fuel to flow and away you go. When you turn off the key the power stops so the plunger extends/closes and stops the fuu
 
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It' won't shut off because the plunger is cut off. That solenoid is the only thing that shuts down the engine. When you turn the key on the plunger retracts allowing fuel to flow and away you go. When you turn off the key the power stops so the plunger extends/closes and stops the fuu
Fuel. The engine consumes the fuel between the plunger and the intake and the engine shuts down. That's why it takes a few seconds to happen. The other added benefit is no backfiring. Witch is why they shut it down that way.

The safety systems will ground the magnetos to kill the spark shutting down the engine a different way. No spark. Like if you get out of the seat etc.

The B&S single cylinder in my old L108 didn't have a fuel shut down solenoid. It just killed the magneto. Backfired every time! That remaining fuel already flowing through the engine doesn't get burned once the plugs stop sparking. Soon as that mix hits the hot muffler PUH-POW.
 

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Discussion Starter #10
It' won't shut off because the plunger is cut off. That solenoid is the only thing that shuts down the engine. When you turn the key on the plunger retracts allowing fuel to flow and away you go. When you turn off the key the power stops so the plunger extends/closes and stops the fuu
Ok... makes sense I guess. I would have thought the key switch would also disconnect the ignition somewhere as well. It can't leave everything energized as you would assume it would run the battery dead.
 

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Ok... makes sense I guess. I would have thought the key switch would also disconnect the ignition somewhere as well. It can't leave everything energized as you would assume it would run the battery dead.
You would think that. lol
That's why the plunger retracts when energized instead of extending. Kill the power kill the fuel.

You can test my theory with the cut plunger. Get the engine running like usual. When you are ready to shut it down activate a safety system. Get out of the seat with the brake off or try to back up with the PTO engaged. Then see if it kills the engine. However the backing up with the PTO engaged will only work with a functioning RIO system. The seat switch must also be functioning. Of course you need to be careful doing either. Let me know if I'm 100% right.

By choking the engine too turn it off. You are doing the opposite of the solenoid. Giving the engine more fuel than it can burn.
 
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The key switch shuts off power to fuel solenoid AND also provides a ground to cut the ignition coil(s). The coils will provide spark as long as the flywheel magnet is passing by it. So to cut the spark the ignition switch grounds the coils. Engine stops..

Sent from my LGL52VL using Tapatalk
 

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Discussion Starter #13
By choking the engine too turn it off. You are doing the opposite of the solenoid. Giving the engine more fuel than it can burn.
Agree. It was sort of a knee jerk solution when it was discovered the engine wasn't shutting off. It was better than grabbing the spark plug. :)
 
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Discussion Starter #14
The key switch shuts off power to fuel solenoid AND also provides a ground to cut the ignition coil(s). The coils will provide spark as long as the flywheel magnet is passing by it. So to cut the spark the ignition switch grounds the coils. Engine stops..

Sent from my LGL52VL using Tapatalk
Never considered that... grounding instead of removing power. I sure appreciate the input. The replacement part arrived this afternoon and he has installed it already. Took longer to find the wrench than it did to install the solenoid. Starts, runs and shuts off now. Should verify the surging issue is corrected during a mow this weekend.
 

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Agree. It was sort of a knee jerk solution when it was discovered the engine wasn't shutting off. It was better than grabbing the spark plug. :)
Almost anything is better than pulling the spark plug wire. With your bare hand that is. lol
Pulling the wire can also cause damage to some ignition systems.
 
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