I got an old riding mower with a 11HP Briggs & Stratton engine. The
mower runs fine most of the time, but every so often it starts to die,
then comes back to a normal idle. However, under load this is worse
and it can totally quit running. When it starts to die, it sounds
exactly like I just shut off the key switch, not like it's running out
of gas. (which has a different sound). I also noticed it does this
more often when I am driving on rough ground.
I changed the spark plug, fuel line (which was getting soft) the gas
filter, and made sure the carb bolts and all other visible bolts were
tight. I also changed the oil, although I know that has nothing to do
with the problem. I also tightened the spark plug wire end that snaps
onto the plug. Then I started it and left it run at idle and in
neutral. It ran for several minutes just fine, suddenly it started to
die but came back in seconds. A minute later is did the same, and
this continued at random. There is no pattern to it. One time it
almost totally died.
Now, while watching it, I shorted out the screw where the wires join.
One wire comes from beneath the engine cover / flywheel. The other
wire goes to the key switch. When I shorted it to ground, it threw a
spark and the engine started to die (exactly the same as the problem).
Now for the question. That wire that goes under the flywheel, I
assume goes to the magneto. While this mower has a battery, I know
the push mowers do not have a battery and have a similar setup. That
leads me to question where is that power coming from. I know a
magneto is a generator of sorts and produces the spark for the spark
plug. But where the heck does that wire go? Like I said, when I
short it to ground, it throws a noticable spark. Is that spark the
same spark that is going to the spark plug? OK, now how do I test the
power at that point to determine if the voltage drops or shuts off
when the engine starts to die? Originally I was going to put a 12V
tester (with built in bulb) across that connection screw to ground.
But after thinking about it, I decided that the bulb would likely fry
in a split second because the voltage is too high. Next, I considered
my multimeter. but was afraid my meter would get fried since it does
not have the capability to test anything over 1000V. So, I put a neon
test light from that terminal to ground. The neon bulb did not light,
so that tells me the voltage is not as high as I thought (the neon
tester is designed for household outlet and wiring testing at 120 -
That leaves me wondering what the heck to use to test that wire.
Does anyone know the voltage at that point?
One other thing I should mention. If I move that wire which goes
under the flywheel, the problem seems to vanish for a much longer
time. Therefore, I am thinking there is a bad spot in that wire, but
without ripping half the engine apart, I can not see that whole wire
since it vanishes under the flywheel.
I'd much prefer to connect a tester before I start to rip the engine
Can anyone help?