314h exhaust pop/shotgun blast after engine shut down

[Matash 15]

Is engine exhaust pop/shotgun sound after engine shut down normal.  I set the low and high end carb needles and it runs great on both levels thanks Matt  

[adsm08 1,434]

A pop as the engine shuts down is usually a back-fire from it leaning out as the fuel mix gets off kilter, they call it a "Lean Pop". A carb cleaning and adjustment may be in order, I have also been told, but never experimented with it, that shutting it down with the throttle in the same position it has been running at helps, so if you have been out mowing and running it wide open, don't throttle back before turning the key off.

[Pullstart 59,733]

I’ve read in the past that carbon buildup in the combustion chamber can glow red hot after a long burn and will help to “diesel” an engine.  It might be a good time to pull the head, clean it up in there and replace the head gasket.  

[Mudrig150 298]

The pop happens because the engine loses spark when the key is shut off, but it doesn't lose fuel. This little detail means that, as the motor is shut off, it's still spinning, so it still pulls fuel through, and pushes it out the exhaust, without burning it.

24 minutes ago, adsm08 said:

A pop as the engine shuts down is usually a back-fire from it leaning out as the fuel mix gets off kilter, they call it a "Lean Pop". A carb cleaning and adjustment may be in order, I have also been told, but never experimented with it, that shutting it down with the throttle in the same position it has been running at helps, so if you have been out mowing and running it wide open, don't throttle back before turning the key off.

On the contrary, you have to run the engine for about 5 or so minutes at idle to help cool down the exhaust, and let the engine spin slower. This lowers the chance that unburnt fuel will travel through the combustion chamber and straight into the exhaust, being ignited by the extremely hot temps. This has nothing to do with the carb adjustment, it just happens. Believe me, I had the perfect settings and my engine still popped after turning the key off.

 

Or, you could add an anti backfire solenoid. It's a little plunger that goes in the carb and blocks off one of the jets, effectively removing fuel when the key is turned off, therefore stopping the popping.

[Pullstart 59,733]

Or that carbon buildup becomes the source of ignition instead of traveling all the way to the exhaust.

[cschannuth 3,795]

 If you were still running at full throttle when you turn off the key almost any engine will do that. If you’ve had it down at idle for a couple of minutes and it still backfires you probably have carburetor issues. I always let my machines run for about 30 seconds minimum at idle before turning off the key. 

[clueless 2,898]

7 minutes ago, cschannuth said:

 If you were still running at full throttle when you turn off the key almost any engine will do that. If you’ve had it down at idle for a couple of minutes and it still backfires you probably have carburetor issues. I always let my machines run for about 30 seconds minimum at idle before turning off the key. 

[adsm08 1,434]

4 hours ago, cschannuth said:

 If you were still running at full throttle when you turn off the key almost any engine will do that. 

 

Mine doesn't.

[ebinmaine 63,156]

Engines can be temperamental and carburetors can have personalities just like a lot of other machinery.

I've experienced both symptoms over the years and I learned when I was fairly young to let the engine idle down and settle at idle for 15 to 30 seconds before shutting off the key or hitting the kill switch.

Back before I switched over to exclusively non-ethanol fuel a couple years ago I was in the habit of shutting off the fuel and letting the lack of fuel kill the engine no matter what it was for a machine. Obviously that had nearly zero tendency to allow that popping back because there was nothing there to burn.

 

 

[Matash 15]

10 hours ago, cschannuth said:

 If you were still running at full throttle when you turn off the key almost any engine will do that. If you’ve had it down at idle for a couple of minutes and it still backfires you probably have carburetor issues. I always let my machines run for about 30 seconds minimum at idle before turning off the key. 

I had throttled it down and then cut the switch off. Then it popped 

[Matash 15]

Would a little non ethanol and a little seafoam help the carbon buildup ?  

[Pullstart 59,733]

1 minute ago, Matash said:

Would a little non ethanol and a little seafoam help the carbon buildup ?  

 

It wouldn’t hurt, but I’d imagine it wouldn’t be an overnight miracle worker either.

[cschannuth 3,795]

6 hours ago, adsm08 said:

 

Mine doesn't.

 Not all do but it’s still a good idea to let your engine idle down for a couple of minutes if you’ve been working it hard before you turn it off. 

[clueless 2,898]

13 minutes ago, Matash said:

Would a little non ethanol and a little seafoam help the carbon buildup ?  

Not sure a little non ethanol and some seafoam is going to do much, but only non ethanol and some seafoam all year long will help for the next 10+ years .

[shallowwatersailor 3,213]

The Kohler Command twins have a tendency to "pop" when being shutoff. My former 518xi would tend to do this, even though I used a treated fuel with Marvel Mystery oil. It is important to not let the engine backfire as it takes its toll on the muffler each time. Eventually it will split at the weakest spot.

 

What I would do is let the engine idle down without a load to start to cool, then as I turned the key off I would shove the throttle to full. This would prevent a backfire. Remember that an air-cooled engine needs air to cool down. Low rpms don't move the air as well.

[EricF 589]

Any hard-working engine will get heat-soaked. It's important to let the engine run for a few minutes after working so that the cooling system has a change to bring the temperature down a bit, and also even out the "hot spots" that develop around the exhaust valves and manifold passages. Even water-cooled engines have this issue. Let the engine idle for a few minutes, and the temperature may briefly rise as the heat-soak works its way through the block and head, but then the cooling system will "catch up" and even out and lower the temperature. Five minutes or so is all it takes. It goes a long way toward keeping the exhaust valves and exhaust system in good condition.

 

This is assuming the cooling system is in good order, of course. For air-cooled engines, it means making sure the fins on the head are kept clean and clear of debris, otherwise the gunk just holds the heat in no matter what.

 

Basically, when I'm done mowing or snowthrowing, I'll just let the engine idle while I clean off the machine. The time it takes to brush off the excess snow and ice in the winter, or to thoroughly wash or blow off the dust and clippings in the summer, is usually long enough to let the engine idle and cool down so it's ready to shut off when the cleanup is done.

[wallfish 15,925]

On 5/23/2019 at 6:59 PM, Matash said:

I had throttled it down and then cut the switch off. Then it popped 

To the contrary of some replies, I shut down at full throttle to eliminate backfire. Try it

Or like shallowwatersailor, move it to full after turning the key off

Thought there was a WH or Kohler service bulletin published for this. Maybe @gwest_ca the file guru can verify it but pretty sure I remember seeing one posted a while back.

 

Isn't it suggested by Kohler and many on here to run your engine at 3/4 to full throttle to keep it lubricated and running cool but then this thread is suggesting running it at idle to cool it down.   I agree a heavily worked engine will be hotter than one with no load so running at no load before shutting down is a good idea. I'm of the opinion to use whatever works for you so if idling down does it, then that works too

[953 nut 51,721]

On 5/23/2019 at 7:20 PM, clueless said:

Not sure a little non ethanol and some seafoam is going to do much, but only non ethanol and some seafoam all year long will help for the next 10+ years .

I'm with Chris on the Seafoam, it will have long term benefits by keeping your fuel system clan and reducing carbon build up. It won't remove carbon build up, but a wooden scraper will. Even if this isn't the reason for the backfire it is a good maintenance habit. You should also remove all the engine tins and clean all dirt and debris from the cooling fins to be sure the engine has an opportunity to receive proper air flow.

Remove the head, be sure both valves are closed and use a scraper made from hard wood, put the head back on with a new gasket and torque according to the engine manual. Once it has been operated to normal operating temperature be sure to torque the head bolts again. 

I always keep the engine RPMs up to half throttle or above until I am about to enter the barn, then run it down to an idle for a minute or so prior to shut down.