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Hi,

I've got a Wheel Horse with a Kohler Command 12.5S that has been running well until recently. All of a sudden with no warning it would start to run rough and backfire through the carb and/or muffler. For a while I would wait for 10-15 minutes and it would start back up. Now it rarely starts. I've done the following:

1. Replaced the plug
2. Checked the fuel path from the bottom of the tank to the carb. doesn't appear to be blocked. Fuel comes out from the line at the carb.
3. Checked the spark with the plug in the hole. The spark isn't blueish white like I';m used to but it's always there.
4. Pulled the carb, cleaned it, and used compressed air on the various areas of the carb.
5. Checked the air gap between the coil and the flywheel.
6. Pulled the flywheel bolt. The key still seems to be in place.

I worked on it yesterday and today. I've noticed the both plugs are carboned on the tip from the small amount of running it does do. When I cleaned the carb I pulled the new plug and put the old one back in. Before I did that I cranked the engine to make sure there was no raw fuel in the cylinder. The air coming out of the plug hole was dry. I put the plug back in and with the blower cover off it started right up. I let it run for 30 seconds and shut it off, restarted it 5 more times and it ran perfectly.

I put the blower cover and air cleaner back on and tried to start it again. All I got was the same backfiring through the carb.

I would appreciate your help in trying to diagnose the problem.

Thanks in advance.

I would have the carb rebuilt, as it sound like you have a rich running condition.

Put up the model/spec. numbers so we know what you have.

just curious, you mentioned "both" plugs??????

I would adjust valves to specs. https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=ADJUSTING VALVES B&S

Note: This is for horizontal crankshaft command pro: section 3 troubleshooting, engine starts but does not keep running:
http://www.kohlerengines.com/onlinecatalog/pdf/tp_2402_a.pdf

http://www.kohlerengines.com/onlinecatalog/pdf/tp_2339_d.pdf vertical crankshaft

Fish said:

Put up the model/spec. numbers so we know what you have.

Click to expand...

It's a Kohler 12.5 Pro
Family: SKH398U162RA
Model: CH12.5S
Serial: 2903308681

The "both plugs" are the old one and new one.

After doing all the things I mentioned I had resigned myself to having to take it somewhere to get it fixed. Yesterday, I let it set for an hour. I got back on it and it started right up (mind you this was after 3 days of not starting). So with high grass I decided to start mowing. After about 10 minutes it quit again. Same symptoms (backfire through the carb and muffler) when I tried to start it. I got the other spark plug and with one still in the head I attached the other one to the plug wire and got quite a good spark. I reconnected the plug wire and still no start. Again I tested with the external plug and this time I got no spark at all. I'm reasonably sure I had it grounded properly.

After another 5 minutes I tried it again and it started right up. I mowed for another 20 minutes with no problem. I took another read on the plug and this time the carbon on the end of the plug was gone and it was clean (at least on the end).

The fact that it runs for 20 minutes would seem to eliminate a sheared key, valve adjustment, and maybe even carb (correct me if I'm wrong). This feels electrical but I'm not sure where or how to start testing.

I appreciate everyone's response so far. Hopefully someone can point me in the right direction to get this fixed.

Thanks!!

Find another tag on the engine, and post the model/spec. numbers., thanks

Fish said:

Find another tag on the engine, and post the model/spec. numbers., thanks

Click to expand...

The spec is 1935
The DSP is 398

Does that help?

Yes it helps know what you have, but eliminates a few possible suspects.

Your engine has hydraulic lifters, so the valve adjustment is ruled out, there is no Smartspark, so that rules that out.
If it ran good without the shroud, it makes me think you might have a chafed wire in the kill circuit, or chafed wires in the harness
that may be feeding partial voltage to the coil. Just for test purposes, unplug the small kill wire at the coil, and put back together and do some mowing with care, as the only way you will be able to shut her down is by choking the carb out, or yanking off the plug wire, which can be hair-raising. Do this to see if it keeps running, and then you can see if the wiring is a factor.

Thanks for the test procedure. I did that, the tractor ran longer but still quit. The first time it started back up within a couple minutes and quit again. I waited a few more minutes and it started up and ran for another 30 and quit again.

All in all it quit 4 times in 1.5 hours. When it does start again there is no black or white smoke. While I was waiting for it to start back up I put a spark plug in the plug wire boot and couldn't see any spark however I was outside.

When it quit the last time I pushed it into the garage, put the old plug into the wire and cranked it. At first there was no spark. As I cranked it some more, there was spark but it was weak. Finally the spark went away, I pushed it in it's spot and walked away.

I appreciate your help!! Where would you look next?

Well if you are saying that you have done all of this with the kill wire disconnected from the base of the coil, and you have intermittent or no spark, then your coil is bad.

And if it is indeed "bad". Then, usually something has killed it... And you need to figure this out before replacing your coil.

What kills the electronic magneto type coils, is "voltage". Stray voltage that gets sent to the kill wire terminal n the coil. So
you need to figure out what is going on. You need to test the kill wire circuit for stray voltage with the key switch, controls, all switches, steering, shifters, levers, etc....

On a normal mower setup, the engine shuts down when the key is turned to the "off" position, which means that the keyswitch
places that connection on the base of the coil to "ground" to shut down the engine. And any voltage supplied to this connection will damage and/or destroy the coil, new or used, so slapping a brand new coil won't do much if it will be destroyed soon.

I simply say replace your coil, as the symptoms you describe are an indication of the coil either opening or shorting to ground when it gets hot.

So you are suggesting that he replace the "bad" coil without testing for voltage to the "kill" circuit?????

But what if he did as you suggest and there is voltage present, what would happen?

Don't know where voltage would be coming from. All his symptoms indicate a coil which is either opening or shorting to ground, caused by heat expansion in either the primary or secondary windings. Never heard of a voltage spike breaking down a coil on a magneto ignition system. Have seen it on a battery system, but in this case it does not apply. Replace the coil is my recommendation, the OP can decide what they would like to do.

Your joking, right?

You have never came across a coil destroyed by voltage getting to the kill wire circuit?

You have to be pulling our leg.....

No, I am not joking. In a magneto ignition system, such as the one on lawn mowers, the only part which produces voltage is the coil itself. I don't know how it could spike itself when it produces the voltage. If it were a battery ignition system, I guess it is possible for the battery to produce current to spike the coil, but that could only occur if the coil was hooked up incorrectly. If you understood how a coil produces the voltage needed to jump the plug, you would understand why I am not joking. If you want to test for voltage coming into the coil be my guest, but I doubt you will find any, as there is no source.

Good Lord!!!!!

Have you ever actually ever worked on anything in the field? You have got to be joking!!!!!!

You have never worked on a tractor with chewed up or molten wiring? A keyswitch that has come apart on the inside?
After all of out thrashing about, you want to go on record for this "stance"??

Cool, because it really shows that you have little or no idea of what you are talking about..... I mean really..... I wanted to make up with you a while ago, but you keep coming back with some real knee slappers!!!!

I am still open to be friends, but I must warn you, when you put out false info, I will have to call you on it. Just because you are old, crochety, brash, doesn't make you right, the same goes for me.

But I hate to be a part of a site that posts total bullcrap, and everyone else ignores it so they won't get singled out.

I give advice to try and help the op, and usually piss off everybody.....

Yes, I stand by my reply. If you can show me how a magneto ignition system coil can get voltage out of a ground, I will change my stance. I am not going to get into a debate over this and the original poster can make is own decision how he wants to proceed, but I am done replying to your posts. Open to be friends, now you are joking, right.

So, your world cannot fathom a wiring harness that gets too close to a steering setup and get chewed up? Or a wiring harness that gets molten by getting too close to exhaust? Or a mouse nest that chews the insulation off of all of the insulation in a
harness????? Or someone replacing a keyswitch?????? Or that someone going to Napa, and getting the "wrong" keyswitch???

Or an old worn out "keyswitch" comes apart inside and sends stray voltage everywhere? Or else, what about a moron trying to bypass everything on his tractor, and he is stupid enough to supply 12volts to his coil, because he is stupid enough to listen to some of the members here?

I am sorry, but if someone is that wrong, I have to say something. I am just worried that you have been teaching as long as you bragg about......

The Result

I ended up getting a new coil and today, I was able to mow the whole yard without the engine stopping. In hindsight, the engine used to require a lot of cranking to get started when it was cold. It was that way for more than a year. With the new coil, it starts right up.

I want to thank everyone for taking the time to respond to my question and sharing your knowledge. You saved me quite a bit of money and I appreciate your help!!!

Rivets said:

Yes, I stand by my reply. If you can show me how a magneto ignition system coil can get voltage out of a ground, I will change my stance. I am not going to get into a debate over this and the original poster can make is own decision how he wants to proceed, but I am done replying to your posts. Open to be friends, now you are joking, right.

Click to expand...

Wow!!!!! Let me see if I can get this post over to that other "coil" thread!!!!

Re: FUEL STARVED NEED DIFF FUEL FILTER OR TRY NONE FOR A TEST XXXXXXX

drivebywire said:

Thanks for the test procedure. I did that, the tractor ran longer but still quit. The first time it started back up within a couple minutes and quit again. I waited a few more minutes and it started up and ran for another 30 and quit again.

All in all it quit 4 times in 1.5 hours. When it does start again there is no black or white smoke. While I was waiting for it to start back up I put a spark plug in the plug wire boot and couldn't see any spark however I was outside.

When it quit the last time I pushed it into the garage, put the old plug into the wire and cranked it. At first there was no spark. As I cranked it some more, there was spark but it was weak. Finally the spark went away, I pushed it in it's spot and walked away.

I appreciate your help!! Where would you look next?

Click to expand...

XXXXXX

FUEL STARVED, CHECK VENT ON CAP, TRY DIFF TYPE OF FUEL FILTER OR FOR A TEST NONE AT ALL