"J Burns" wrote in message
...


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!


When I bought a Huskievarnia riding mower about 3 years ago I was told by
the service man at the dealers to always turn off the key with the engine at
full throttle. He mentioned something about it being bad for the engine,
but I forgot what it was. Never had a backfire doing that.