The Stovebolt Site Forums The Shop Area Engine Shop Is it the ethanol?

Today was a nice day, temps in the upper 70's.
I got one of the John Deeres out, and did some brushogging. Noticed that every time I shut it off, there would be a loud backfire, after turning the ignition off.
My lawnmower does the same thing.
My 57 GMC LCf backfires all the time, when you let off the gas, going downhill. But I don't have a muffler on it yet, just a short pipe.
Do I need to try some premium or non-ethanol gas?

no

Backfiring on deceleration is often a sign of a lean condition. I suspect on your Jimmy it's because of no muffler. It could be a carburetor problem or intake manifold leak or other issue. I seriously doubt the ethanol has any role in your issues.

Thanks. None of them do this until they are worked hard and engines are hot.
I was thinking timing, but I hate to mess with them. They start good and idle good and actually run good.
The tractor will scare the he** out of me. It will go off like a 12 gauge, just about the time I get off the seat.

The very hot exhaust valves and or pipe is most likely lighting off the fuel still remaining in the system. Carburetors continue to feed fuel as long as air is pulled through them, most of the time it just goes through the engine with out any problem. Try letting them idle a little to cool off the hot pipes. Lean burning makes for hot exhaust but also for a more efficient running engine.

Joe

Joe I got a kick out of reading your post.

My father always used to make me idle our Cub Cadet lawn mower for a minute before turning it off. 50 years later I still follow his teachings.

Dad never explained it eloquently, as you did, but he no doubt understood the reasoning for doing it.

John

Letting a motor idle for cool down is just that.. not specifically for any fuel/backfire related issues... just to simply let the heat get taken out of the motor prior to ALL cooling being removed (aka turning it off).

Even here you'll find frantic posts about the water temp gauge shooting up past 200F after they turn off the motor. A 2-3 minute cool down negates that... but it is not required.

Yeah, I thought it was heat related, because they don't do it cold.
I will start doing the idle thing now. Thanks to all.
Matter of fact, now I remember in the old days, some cars you could turn the engine off quickly, after a hot run and they would keep running for a few seconds. We called it running backwards.

Lots of different issues will cause a backfire (or series of backfires) in the exhaust on decelleration:

(A) carburetor too rich
(B) carburetor too lean
(C) cooling system marginal
(D) camshaft with lots of duration
(E) sticking distributor advance
(F) other

The ethanol MIGHT contribute to either B or C, but as others have mentioned, probably not the issue.

Incidentally, my John Deere 420 did the very loud single backfire about 30 seconds after shutdown (idling did not help, but a carburetor rebuild did). The carb was partially plugged by the evaporation of modern fuel which contributed to C. And I detest working on small engine carburetors .

The carb rebuild completely solved the issue.

Jon.