What causes run-on and backfiring on shutdown?

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exhaust leak at the head, clogged shredder, broken or clogged fan, oil soaked engine, lean carb/air leak behind the venturi, and carbon knock. You may have said earlier, but how many hrs on the engine. Cylinder should be decarbed at 500 hrs and new head gaskets installed. It's rare to find someone who will do that at 500 hrs. Usually they wont do it till something goes wrong.
 
Originally Posted By: 660mag
exhaust leak at the head, clogged shredder, broken or clogged fan, oil soaked engine, lean carb/air leak behind the venturi, and carbon knock. You may have said earlier, but how many hrs on the engine. Cylinder should be decarbed at 500 hrs and new head gaskets installed. It's rare to find someone who will do that at 500 hrs. Usually they wont do it till something goes wrong.



Clogged shredder?? Is that the fan screen?

Of what you listed, the fan is clear, the fins seem clean (from what you can see), the exhaust seems fine (no obvious leaks), and the engine is really clean. Not sure about the carb running lean (it starts fine, runs fine, idles fine, and accelerates fine), or the carbon issue.

The engine has 650 hours...it was supposedly routinely maintained by the shop it was purchased from, if the PO was honest with me.

My father recommended a fogger from OMC that is specifically for removing carbon from engines. He said it works extremely well.
 
I just read the manual, there doesn't appear to be any adjustment for fuel, short of a jet change for altitude...only idle fuel can be adjusted on certain models.

My next steps are:

Re-gap/replace the plugs.
Verify the cooling fins are clean/clear.
Run some carb cleaner through the system.
Do something to remove the carbon.

I can hardly believe there is an issue with either the plugs or the carb as well as the thing runs...
 
Someone mentioned that the fuel might be old...the fuel was less than a week old and had stabilizer, for the record.

I run 89 in my Superhoopty, so it would be easy enough to grab a can full to try.
 
Yea it's the fan screen, same thing.
From what you say it does not sound lean to me.
Chemical or water decarbing works to a degree, but nothing beats cleaning by hand. Not sure I'd call decarbing routine maintenance as a twin takes several hours to do it well. I lap the valves too when doing a decarb. It doesn't take much extra time, and they run so smooth when it's all done.
 
You said the previous owner just changed the plug. I wonder if he installed a hotter plug (i.e. a plug with an hotter operating temperature). You might get a plug hot enough that it would cause detonation after switch off.
 
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I pulled the plugs tonight. They appear new and they are the correct plug for the application. I didn't have my feeler gauges handy, but they were gapped equally...but they might be gapped wider than 0.030". I will dig out my feeler gauges this weekend and check...I'll bet they were never "gapped" past the Champion OE set, which is typically 0.035" IIRC.

I pulled the air cleaner and sprayed a bunch of carb cleaner through it...when I first pulled the air cleaner cover, I found that filter had been leaking. I have replaced it, but was told I should spray the carb down...it was pretty dirty.

It runs the same.

I did shut it down each time at about half throttle...not a pop, not a run-on. That might be the answer.
 
Originally Posted By: demarpaint


You can also do a piston soak with Seafoam, or MMO. IMO if it is hard carbon the water mist I mentioned is the better option.


...or Amsoil Power Foam...
 
I pulled the plugs and found they were gapped somewhere between 0.040" and 0.045", so I set them at a tight 0.030". The plugs were brand new, and they are the version recommended for this engine.

We got over two inches of rain last night, so it will be a while before I get out to try it...it starts the same. I have also not had a single backfire since I've been shutting down under partial throttle.
 
We had it out last night for about an hour, and it is running cooler...I can actually touch parts of the shroud, and the oil filter and oil cooler aren't screaming hot (they are hot, but not sizzling like before).

So adjusting the plug gaps seems to have worked!

The Wife looked like a drunken monkey trying to cut a straight line, and I thought she mowed of two bushes (near misses)...but she was doing pretty good at the end.
 
As a follow up, by shutting down at part throttle (fast idle) and regapping the plugs; I have not had a single backfire episode since.
 
Originally Posted By: deeter16317
As a follow up, by shutting down at part throttle (fast idle) and regapping the plugs; I have not had a single backfire episode since.


What happens now after re-gapping the plugs if you shut down a idle? Just trying to separate the effect of re-gapping the plugs versus shutting down at part throttle for future reference.
 
It seems to shut down normally...but we just follow the advice given and shut down under part throttle to be sure.
 
Originally Posted By: deeter16317
As a follow up, by shutting down at part throttle (fast idle) and regapping the plugs; I have not had a single backfire episode since.


Good deal. The weirdest thing about this issue is, some Kohler Command twins do it and others do not. It seems like it depends on how the engine and exhaust system are configured. I've always theorized it's due to the exhaust, with the last blip of unburnt fuel vapor lighting off in the hot exhaust after the ignition was killed. Throttling up leans the mixture out enough where it won't light off. It's been my experience that it mostly happens on horizontal shaft Command twins. Not so much on the vert. shaft ones.

Joel
 
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As a follow up...

Although I hadn't had any backfiring following the shut down instructions given in the above posts, I added some seafoam to the gas and poured some in the carb body (while running) as a preventative maintenance thing...that was back in June. We've put probably 25 hours on it so far...here's the kicker: we can shut it down at an idle and it will not pop or diesel. I'm thinking between the fresh HDEO (Schaeffers 5w40 Series 9k) and actually running this thing for an hour at a time WOT, it has cleaned itself up.

I haven't had any further issues with it.
 
The B&S 190cc "8.75" Intek in my 4-year old Snapper had recently been smoking a bit on startup, and more recently started to backfire (not a pop, more like a .22) after every shutdown.

I had changed the sparkplug late last year, but didn't think to check the gap. I'd several time knocked the dust out of the air filter and washed the prefilter, but hadn't ever managed to buy a new air filter yet.

So I finally managed to coincide remembering the filter, being at an appropriate store, and knowing which filter to get.

I used some carb/intake cleaner on a rag to clean the debris off where the filter seal sat, and since I'd been playing with Seafoam on cars recently, I poured a little down the airhole under the filter. I sucked it through gently by pulling the starter a little, and then ran the engine briefly to clear it out.

This reminded me of looking at this forum about Seafoam on lawn mowers, and I saw B-12 recommended.
I got some the next day, took the filter off again, and poured rather a lot into the air hole.

It was so much that I couldn't pull the starter much, so I took out the spark plug to relieve pressure.
(I also checked and set the gap at this time, which had been .030 instead of the required .020.)

The first slight pull sprayed a bunch of B-12 out of the plug hole, so I put a paper towel in front of the hole.

(B-12 strips paint fast. How do you prime a mower deck?)

After letting it sit a little while, I pumped out the remaining wet B-12 by pulling the starter. On every pull, vast amounts of tiny carbon chunks sprayed onto the paper towel which I kept replacing to see if it was done yet.

It seems to start a little easier now, and after a while it occurred to me that it's no longer backfiring!

The air filter had previously been dirtier than it was just before replacing it now, and the spark plug gap had been off for much longer than the backfiring (but was causing the startup smoke, perhaps?), so I'm guessing that the cleaned-out carbon (which might itself have been caused by the gap or the dirty filter?) fixed the backfiring.
 
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