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Hi, I'm new to the forums, but have been browsing looking for answers for a couple of days.
I have a 20 HP two cylinder Briggs and Stratton with dual magnetos. When I first tried to start it this year, it wouldn't start. I thought it might be gas, so I put in some new gas, tried spraying some ether, and a couple of other things, but it wouldn't start. It appeared that I was not getting spark on the plugs so I replaced them. After that, it started and seemed to be running fine, so I prepared to mow the lawn. Then after running for about a minute and a half, it just stopped. After that, it wouldn't start again. I took off a magneto lead to test the voltage and it showed 12ish volts when cranking. Surprisingly, the mower also started. I then tried plugging that magneto back in and unplugging the other one. Again, the mower started. Plug them both in, won't start. I'm pretty sure that when the magneto is unplugged that guarantees that the cylinder doesn't fire, but the mower did not even seem to be running rough.
Anybody have any ideas what I might have going on?

First I would check to see that your kill isent grounding out, you should be able to run it without your magneto hooked up

RoperGuy said:

First I would check to see that your kill isent grounding out, you should be able to run it without your magneto hooked up

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Ah, I thought the wire to the magneto actually powered it. I didn't realize it was a kill switch. OK, I checked while cranking and in both cases it indicated open connection. When I put it on 20k ohms, I was getting occasional readings for connection to ground. Like if I cranked for three seconds, it might flash away from open connection to showing a few thousands ohms resistance once.

When measuring voltage between the kill wire and ground, it registers about 1 Volt while cranking.

V Twin B&S engines have a weird Kill wire circuit containing diodes which sometimes go bad causing some unusual symptoms. IF you unplug the Kill Wire from the base of both coils and all seems well, then you need a new Kill Wire wiring harness. Note that you will need to shut off the gas or some other means of stopping the engine after test.

Walt Conner

Tinkerer200 said:

V Twin B&S engines have a weird Kill wire circuit containing diodes which sometimes go bad causing some unusual symptoms. IF you unplug the Kill Wire from the base of both coils and all seems well, then you need a new Kill Wire wiring harness. Note that you will need to shut off the gas or some other means of stopping the engine after test.

Walt Conner

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Unplugged the kill wire from both coils and it ran like a top. Also, removing the key was sufficient to get it to stop. Perhaps there is a fuel pump or at least a valve that gets turned off when you remove the key.
I guess I need a new harness. Figures that this would happen on the one year where I actually stored the mower in the garage instead of sitting outside.

ptomko said:

Unplugged the kill wire from both coils and it ran like a top. Also, removing the key was sufficient to get it to stop. Perhaps there is a fuel pump or at least a valve that gets turned off when you remove the key.
I guess I need a new harness. Figures that this would happen on the one year where I actually stored the mower in the garage instead of sitting outside.

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Obviously it is equipped with an anti-after-fire solenoid which shut off the fuel to the main carb jet.
stopping engine.

Walt Conner

For the mean time you can install a switch between your kill lead And ground, you will manually have to flick it on and off