On 8/21/15 7:57 AM, Arnie Goetchius wrote:
J Burns wrote:


Does yours backfire when you shut it off?

I'm getting intrigued. The only shutoff solenoid I was familiar with, was on the
Kohler engine of a John Deere riding mower from the late 80s. If it was to
prevent backfiring, I wonder why earlier engines didn't backfire when shut off.

I have a Simplicity from the early 90s with a Kohler engine. The carburetor
looks identical to carburetors with shutoff solenoids, but this carburetor never
had one. It was 20 years old when I got it. For the first year, it would
backfire perhaps ten seconds after I shut it off. For some reason, it hasn't
backfired in a long time. Whatever caused it to backfire, it doesn't need a
solenoid to prevent it.


I never had a backfire before or after installing the shutoff.

Strictly speaking, backfiring in the exhaust is afterfiring. Gas
self-ignites at about 500 F. One would think that when the ignition was
cut with the engine not under a load (small gas/air charge), each little
charge would ignite as it left the exhaust valve, without much noise,
like a blowtorch.

Maybe it doesn't ignite because it mixes with the exhaust in the
muffler, diluting it too much to explode. Maybe after ten seconds, the
cooler oxygen and gasoline settle to the bottom, reaching the
concentration necessary to explode.

How did afterfiring become a problem? The EPA! It used to be routine
to throttle down all the way before cutting the ignition, but for
decades, manuals have said not to throttle down. I think I see why.
It's hard to design an gas engine to burn clean at an idle. If engine
companies said not to idle, they could give the EPA emissions figures
that didn't include idle speeds.

If the ignition is cut at an engine's working speed of 3600 rpm, the
inertia of the engine will probably pump 16 times more air/fuel mixture
into the muffler than if the operator had first throttled down to 900
rpm BAM!

The reasons to run a mower at maximum rpms would be for blade power to
cut heavy growth, for wind to prevent clogging, and for ground speed to
race through a job. I don't have a tach, but I suppose I've generally
run at half speed for some time because full speed isn't usually
required. A slower engine means less noise, less dust, less fuel, and
less slowing for turns. Shutting off at half speed means much less
fuel/air in the muffler. That would explain why I haven't heard a bang
in a long time.

I believe I will now ignore the manual and throttle down all the way
before switching the ignition off. It couldn't cause much pollution
unless I idled extensively. What kind of guy idles a mower extensively?
The kind who takes cigarette breaks! He goes next door for a moment,
and all the children die of terrible diseases, and statisticians
attribute it to third-hand cigarette smoke, the smell on his clothes.
In fact, they died because they were downwind from his idling mower!

I'm starting to think consumers were saddled with expensive,
trouble-prone shutoff solenoids simply to pass EPA requirements that are
meaningless in the real world. It would be great if a solenoid could
prevent trouble from a leaky float valve, though!