View Full Version : Diagnose Kohler engine problem...

03-01-2001, 07:58 PM
I have an Exmark Lazer Z w/ 25H.P. Kohler Command vertical crankshaft engine. It has been a very reliable mower. I have app. 1400 hrs on engine. Several months ago, I noticed that very infrequently, the engine would start to bog down momentarily-this only happens about 5% of the time. Usually it happens when it's shut down hot and sometimes commences when I first power it up again after shutting down for a short time. It's like it isn't firing on both cylinders, sort of like it's not getting enough gas and a small amount of black smoke comes out of exhaust. After about 20 seconds or so it begins to gain R.P.M.'s and runs great, black smoke disapears. Like I said, this happens very infrequently but I wonder what is causing it. It does NOT burn any oil. I took it in to the dealer and he replaced plugs, etc. It still does it. I guess I don't blame him, it's one of those things like with a car or truck, if it does not run poorly 95% of the time and the dealer does not notice any thing wrong, he figures everything is o.k. Could this be a carburator adjustment/fuel mixture problem? Or head gasket problem? I just don't want to ignore it and something major happen. Any thoughts would be appreciated.
P.S. I'll take it back to dealer when I get thge time, I'm hoping some of you guys can give me some idea to tell the dealer to check.

03-01-2001, 08:17 PM
I would tend to lean more towards the carb end of the scenario. It doesnt sound like an adjustment though due to the fact that it does it so infrequently. And assuming that you have changed the fuel filter and checked the carb for lodged debris i would stand on the air/fuel mixture. I am no certified mechanic but a desent back yard one and this is my opinion.

03-01-2001, 08:32 PM
When you shut it down, you have to let it idle down for about 30 seconds or so first. This allows the fuel in the bowl to go back down and allows the float to settle. What's happening, basically, is that your engine is entering the beginning stages of flooding out. That's the black smoke you see when it is first fired up again. The sputtering or hesitation is from too much fuel in the cylinder(s) as well. Does it backfire on you when you go to restart it? This is not that uncommon when it is not allowed to idle down too. What I do, as a time saver, is when I put it up on the trailer, I brake it, get off the machine, and put the gate up while it is idling. Then, I just reach up to shut the machine off. This idling time is then better utilized and not wasted just sitting there waiting for it to idle for the needed time. Try it and see, and I think you will find that this will probably take care of your problem.

Jerrys Lawn Service
03-01-2001, 08:32 PM
Bassman,check with a new member, Gorfis he says he's been a small engine man for 7 years and would be glad to try to help with any questions.

Good Luck!

GreenQuest Lawn
03-01-2001, 08:39 PM
Try to replace the hydro filter (if you havent already). Mine did the exact same thing only i noticed when i went up hills with the blades on. Replaced everything else first but ended up being the hydro filter. (and it was only 150 hrs since last filter change) so i have started changing it more often.

03-01-2001, 09:00 PM
it sounds like it may be loading up, getting to much fuel. You should have the carb rebuilt, and ask them to check for wear on all orfices (from additives and cleaners). If this does not resolve the problem, do a compression test with leak down, you may have a worn valve or be spinning a ring.

03-01-2001, 11:20 PM
It sounds to me like you are getting a vapor lock problem in the carb while it sits for a short time while hot. Poorer quality fuels we have to use now a days contribute to that. Also I have a suggestion for shut down. You guys are using air cooled engines, they get their air from the fan pulling into the engine that is located on the flywheel. The slower you Run the engine the less cooling air you get to it. My suggestion is to run it wide open, NO load. The throttle plates on the carb are not open and you get all the rpm needed to keep the fan going at speed. Also on the Kohler engines they need to be shut off at half throttle or higher. They have a fuel shutoff solenoid in the high speed circuit of the carb. If you shut them off at idle you still pull fuel in to the engine fromthe idle circuit and you can end up with brown seat sindrym when you start climbing off (after bang). I shut my Kohler units off at wide open throttle. They are constantly getting oil from the oil pump and lubricating all surfaces and no fuel is getting into the engine to wash the cylinders down. These are just my opinions.

03-01-2001, 11:43 PM
Thanks for the replies. From the posts so far I think I'll either let it idle for a few moments before shutting down and/or also try shutting down with throttle at high end. If this does not stop it, I'll try replacing the hydro-filter.
As I stated, the overall performance is fine. I just notice this when it's hot and I've shut down and then restarted a short time later. I have always idled down after mowing and then immediately shut down, like right away. No biggie, mower runs great once I'm powered up. Thanks for info so far. More replies appreciated.
P.S. My engine has sometimes diesels just a tad when quiting and often when it does not diesel it shuts down with a pretty good backfire. Is this normal?

[Edited by Bassman on 03-02-2001 at 04:03 AM]

03-02-2001, 12:25 AM
No, a backfire is NOT normal. Sooner or later, you are liable to blow your exhaust off. I have done that before. Just let it idle down a bit(give it a good chance-atleast 30 seconds or so) and then shut it off. I will bet a dollar to a donut this will cure your problem. As a matter of fact, PLEASE let us know how it turns out. Remember this thread, and post on it. It will come back to the top. :)

John DiMartino
03-02-2001, 12:54 AM
There is nothing wrong with letting your 25 idle for 30 seconds before you shut it off.The fan may turn slower at idle-but the engine isnt generating hardly any heat either with no load,and idle speed,friction is low,as is fuel usage which keeps combustion temps down,and My 25 is not a Command pro-the pro is the only one with the fuel shutoff-if you dont have a pro-you should be idling down before shutting it off.If you dont and let it wind out,the exhaust is hot at wide open,you shut if off,the governor is mechanical,it opens the throttle blades and dumps fuel into the engine,the fuel goes thru the motor and blows up in the muffler which superheats after shutdown.I feel the problem with Bassman's is the carb leaking down,either from a high float level,leaky needle/seat or loose bolts,gaskets.The motor is running rich when its blowing black smole-this is no good for it at all,and washes the cyl down,and dilutes the oil with fuel.The easiest thing to do it buy a new carb and bolt it on.

03-02-2001, 09:06 AM
All Kohler Command twin cylinder engines have the fuel shut-off solenoid, not just Pro series. Some of the single cylinder engines have them. They need this due to the large displacement of the engine as to not pull fuel in to the engine and dump it in the muffler. I too agreee you will damage the muffler if it is allowed to afterfire. So I still recommend shutting off at high speed. Would you pull a $30,000 pickup into the driveway after a 500 mile trip, drain half of the anti-freeze out and let it sit and idle to cool down?? When an engine is shut off you get an effect that is called heat soak. The engine temperature actually rises for a short time higher than when it was running. The thing to remember with the deck off and not using the hydro to move the equipment the throttle is barely off of the throttle stop screw. Very little gas getting into the engine.

03-02-2001, 10:07 AM
If the engine has a fuel shutoff solenoid on the carburetor then you definately want to shut down engine with half throttle or higher. When you turn off the key the engine will spin over a few more times to clear out fuel left in the cylinders and muffler so you won't get backfire. If it doesn't have a shutoff solenoid then idle it first before shutting it off. As far as the startup, no power problem, I would ask if the engine sounds weird like air escaping when it's idling (possible blown head gasket). You should definately hear it if it is. Next how old is your machine? The early commands had ignition coil problems. Remove the blower housing and look at your coils, if they're marked MA2A or MA2B then get it to your dealer immediately, these coils have to be replaced with newer ones (Kohler will warranty it). They should be marked MA2C or MA2D. You can have your dealer do a quick check too to see if the spark advance module is doing it's job of advancing timing correctly. Don't overchoke engine either if it's already warm. This is about all I can tell you from here.

03-02-2001, 10:39 AM
I had new ignition coil put in just about 6 months ago. There seems to be two schools of thought on shuttin this model engine down. One says let it idle for a bit first, the other says to shut down from high throttle. I will try both of these today and post my findings.

03-02-2001, 11:00 AM
The answer to the question is:

25HP Kohler!

Sorry for being a smart bAss but, ah crap I'm tired of talking about these engines.

Really Bassman, Good luck as I have had nothing but grief with the one I had. You had 10 times more hours on yours before you started having problems. My advice that is only an opinion is to trade it now before you get busy. Good Luck.

03-02-2001, 08:19 PM
Good news and bad after trying shutting down different today.
After mowing and engine is hot, I tried letting it idle for a couple minutes, then shut down. I still experienced dieseling.
Then I tried shutting down the rest of the day with throttle close to wide open. No dieseling and no backfire the entire day. That's thre good news. By the way, thanks Jman for the tip.
The bad news is I still got a bog down on restart with engine hot, (sounds exactly like engine is flooded), for about 20 seconds.
So, I now know how to keep engine from backfiring and dieseling at shut off. Shut it down with wide open throttle.
I guess I have a carb problem. I did notice that the sympton is more severe when blades are engaged and/ or going uphill, so maybe the suggestion about hydrostatic drive filter has merit.
Answer to Mow Ed, This mower and engine has been very, very reliable. Even with this problem, once I gain r.p.m.s after about 20 seconds, it runs like a champ. Once again, it's only when it's hot that it does this. I'm sure my dealer will resolve it, especially with the info I've picked up from you guys. I am not about to get rid of the mower. I feel like it's a minor problem and considering I have over 1400 hrs and this small quirk just now showing up, I really can't complain.When I isolate what is causing this I'll update you guys, it may help someone else with similar symptom.
Thanks to all.

03-03-2001, 02:44 PM

you said you had the ignition coil replaced. did they replace both of them? there is two of them. did they also replace the S.A.M. box? (spark advance module) Also they have a lifetime warranty on all those ignition parts.

03-03-2001, 11:10 PM
Are you sure about the lifetime warranty on ignition parts? I paid for my SAM and ignition module.

03-04-2001, 12:43 AM
Yes, all ignition components using the S.A.M. are lifetime warranty.(the sam and both coils)

03-04-2001, 08:42 AM

Sounds like you got shut off at idle cause they POPPED you! I have had coils replaced on everything BUT my 25 Kohler and never payed a dime for it, yes there is a lifetime warranty on al ignition parts.

This advice about turning the key off at WOT is new, we have all stated in the past discussions to shut it down after it has idled. Now I guess I.m confrused :confused:!

Since I am no mechanic and really have no desire to be, I have 3 Kohlers, 1 is an 18, 1 is a 22 and the other is the 25hp. The 25 has a wire running to the back of the carb., what is this? Which one should be shut down at half throttle and which one should I idle down. This is way too confrusing for a small minded guy like me. :rolleyes:

03-05-2001, 09:46 AM
In relpy to Homer. If there is a wire going to the carb on your Kohler engines, it does have a fuel shut off solenoid in the carb. This shuts off fuel in the high speed circuit only. The old rule of thumb for years has been to shut off the engine at idle after letting slow down for a long period of time. Times have changed now. As long as there is a wire to the carb, shut off at WOT.
In answer to the lifetime warrenty. ALL Kohler engines that have Smart Spark ignition have a lifetime warrenty on the ignition system, provided it is replaced by an authorized Kohler dealer. If you do the repair yourself, they do not pay warrenty. As a tip, if you do repair your own engines the recommended coil air gap has changed from .010 of an inch to .012 This helps eliminate some cold misfire issues that occured from the module being mounted too close to the flywheel.
Bassman, Is your heat shield over the exhaust system all intact, or is it starting to break apart? If anything is missing it can still cause the carb to get extra hot and could be boiling the gas in the carb after shut down. Also, change fuel to a higher octane level for one tank full and see if that makes a difference. I do agree with John carb overhaul, or replacment may be needed with the number of hours, but check exhaust heat shields first.