12 hp kohler backfire on shutdown

[k. hansen'49cub]

Hello All,
One of my Cadet's had a recent rebuild ,now when I shut it down it sound's like a rifle shot ,the shop manager insist's it's "normal"...Then how come it never did it before the rebuild? and how come none of my other cadet's have that problem,to me it sound's like he timed it wrong,if that were the case would it run?he tell's me it would not,he insist's it's the carb ,the carb is tight,anyone have a suggestion? I have no plan's on bringing it back to him,I will have to take care of it myself. Thank You.

[Eugene]

Timing could be of by a degree or two. Check with a timing light.

Suggest letting the engine cool off, idle for a minute or two before you shut it off. My wife has the engine wound tight then turns off the key. The engine will pop. I suspect its the unburned gas expelled from the combustion chamber being ignited in the exhaust pipe/muffler.

Eugene

[Eugene]

One more item to check after the timing. Make sure that the throttle plate in the carburator is closing fully - minor adjustment on the throttle cable.

Eugene

[k. hansen'49cub]

Thank's Eugene,
I give it a try, I've tried to let it cool ,idle for a few minutes,it just prolong's the bang ..really I'm to the point where I don't even want to use it anymore,one way I shut it down without the backfire,is to full choke it ,and let it die.
Thank's for the tip's !!

[Paul B]

You didn't say what model the tractor is, but if it is, say a 1320, (12 hp vertical shaft Kohler) the pop is rather common, and if I remember correctly, the ops manual says to let the engine idle a bit before turning the key off. EPA rules and regs show up in some strange places, even in lawn and garden tractors, and that backfire/pop is the result of trying to comply with some of those regs.

[wdeturck]

That pop had a name that I wanted to remember but I got CSR. Some Kohler engines seem to do it a lot and it comes up quite a bit on the forums but I never heard of it causing any damage to the mufflers, etc.

[johnbron]

If this engine is fairly new it probably has an electricly controlled Carb on it (Thanks To EPA). For those I have had good luck shutting them down at 1/2-or-3/4 throttle to eliminate the "Ka`Pow". If you can find an old~style Carb with adjusting screws swap it with the carb on it and you can adjust that Pop out of there usually.

[Dave (69 lo-boy)]

The Kohler owners manual is very specific about bringing the engine to an idle and letting it run at idle for 30 - 60 seconds before shutting off the engine.
When the engine is running fast it continues to pull a lot of fuel in after the ignition is shut off, a piece of hot carbon in the cylinder or the exhaust will ignite the fuel in the exhaust, makes a load pop, and may even destroy a rusted muffler.

[k. hansen'49cub]

It's the 301a from my '68 125 Hydro, my manual state's allow engine to idle for a short time before stopping,I have always done this any way with all my tractors,After the rebuild ,I could let it idle for 5minutes ,it still goes BANG,like I said before ,the only way I can get it to stop without is by full choke,and let it die. I just don't understand ,none of my other cub's pop,and before it was rebuilt ,the 125 never did it either,perhaps it's the new ring's?,the old engine did'nt burn oil,what happened was the rod locked up on the crank, this guy rebuilt the engine,I put about two hrs. on the new engine ,it did the same thing,only this time it ruined the new rod, so this guy wanted to put in another rod,I said no ,I brought him my spare 301a to install ,what this guy did, and I was FURIOUS, he dismantled my good spare ,and in stalled the rod in my old engine, so needles to say ,I have alot of part's, I should have just swapped out the spare myself, what if there is something wrong with the crank in the original engine.....?then that mean's this moron took apart my good engine,and eventually it will happen again,perhaps I should get a rod,and piston for my spare part's and keep it in reserve.

[Dave (69 lo-boy)]

I would be furious too!
You can download the service manual for free at:
http://www.kohlerengines.com/onlinecata ... m=K-Series
It could be valve adjustment, carbon buildup or sump'm like that, the service manual does not say anything about backfiring, so they offer no hints.
Does it look like it is running rich, is the exhaust sooty?

[John *.?-!.* cub owner]

I see a lot of posts aobut this problem with Kohlers on Cadets and other tractors. Seems to be a common problem. Used to have an Allis with a 16hp Kohler that would give you enough time to start to get off before it did it. Really loud in a 10x10 metal shed. I have since seen several posts stating that if you shut it off while it is at about half throttle it will solve it.

[Paul B]

I assume from your post, that you do not, for what ever reason, work on engines. If that is so, you need to find someone else to do your work - I would not have gone back to him after the first problem, because I do not think he knows much about them. Are you sure you are getting "your" engines and that he is not switching them out ? If he did in fact rebuild the engine, there should not be any carbon buildup etc., to cause the problem. I believe the problem lies in the carburetor adjustment - too lean, to rich, worn throttle shaft, clogged jet etc. If you had a good one going in, he may have switched carbs on you. If it is caused by carbon buildup around the valves (cause sticking exhaust valve) it tells me he did not know what he was doing if he actually was inside the engine. You need to find a dealer or good small engine shop to look at it.

[k. hansen'49cub]

Paul,
The problem is I work on engines all day, Detroit,Cummins ,Navistar, I dont mind working on my Cub, or my 8n, or the B ,the problem is I dont have the time to spend on a learning curve on the small engines ,I admit beyond the carb adjustment's , a small eng is alien to me, perhaps I'm sort of ignorant toward's them ,I thought I'd try the expert's who have been around these thing's ,perhaps there was a quick fix ,at this time I have too many project's in the work's to get diverted on learning a small engine,I've never had one apart,I figured it would be better just to drop it off, and pick it up finished, but it look's as if I will have to spend the time to do the job right ,I guess all of that don't make much sence to some of you,but I have alot on my plate, and I'm sure once you get to know these engines,it's probably easier than the stuff I work on all day.

[k. hansen'49cub]

If you had a good one going in, he may have switched carbs on you. If it is caused by carbon buildup around the valves (cause sticking exhaust valve)

That is what I initally thought that the exhaust valve was hanging open ,but I assumed that if it were rebuilt,there would be no carbon in there,the carb I readjusted ,but the throttle shaft seems to be tight ,no wobble ,that's why I'm sorta puzzled ,I don't know enough about these small eng's to just say "that's it" at current I have one car ready to go to the paint shop ,one car I'm completely reconstructing the panel's from the door's back,and it's going real well ,also I only have room to work on the two car's in my garage ,so that's part of the reason I did'nt bother.I appreciate the advise you guy's alway's help with answers.ThankYou.

[Paul B]

Believe me, I can fully understand your reason for not working on the Kohler yourself. I sometime think the older vehicle engines (without all the computerized controls and gizmo's on them) are easier to understand and repair than small engines. What I can't understand is if your engine ran good before the rod failed, and doesn't now, there is nothing about changing a rod and piston that would cause the problem unless other parts have been changed/switched also, unless maybe if the crank was out of the engine, it is out of time.