Engine shutoff without backfire.

[Gravy]

Hey folks,

I haven't been around much for quite a while, but a recent thread reminded me of a problem I solved quite a few years ago: The dreaded backfire a few seconds after shutdown. Maybe somebody else already did this, but I'll go ahead anyway.

It occurred to me that turning off the ignition on a hot engine allows the engine to spin down, drawing fuel/air mixture through the carburetor and pushing it through the combustion chamber where there is no spark to ignite it, and into the glowing hot exhaust, where it would ignite and go BANG. I decided to approach it from the fuel/air end by shutting off the fuel first and leaving the spark running. All it took was disconnecting the wire from the M terminal at the ignition switch. When turning the key off, the spark plugs keep firing, but the fuel shutoff solenoid closes and the engine runs out of gas in a few seconds. No more bang. I did wire in a separate kill switch for the ignition, but I have never had to use it, and it's been quite a few years.My nerves and ears are are much happier.

I hope that helps someone.

 

[m5040]

IMHO you should idle the engine long enough to disperse the heat, so you can shut it down without a backfire or dieseling.

 

[scaredychicken]

I get a similar but rare backfire in my El Camino ... in that case, it has been suggested to me that the timing needs to be adjusted

 

[m5040]

He explained how it happens. There is no spark, so how would timing come in to play?

 

[scaredychicken]

m5040 - i was referring to my vehicle's timing and occasional backfiring, not suggesting that it was the same for the tractor. I'll clarify my comment

 

[Chilliwack Murray]

We have a newer push mower that does that, no way to idle it down or shut off the fuel before the spark. Usually does it a full second after it stops and gets the dogs barking for miles around.

 

[MuncyBob]

I find that when I can run ethanol free gas I never get that backfire.

 

[lpakiz]

I had always thought that the "backfire" was ignited by a piece of red-hot carbon stuck to some part of the combustion chamber or exhaust valve.

 

[m5040]

We have a newer push mower that does that, no way to idle it down or shut off the fuel before the spark. Usually does it a full second after it stops and gets the dogs barking for miles around.

That is because the government emissions standards can't be met if the engine went to idle. So you shut it off at high rpm. Nobody cares how long it will last, you have no choice but to buy a new one (or learn how to move the carb throttle to a low speed for a short while before shutdown).