Winter has arrived, which means it’s time to winterize the outdoor power equipment for the season to keep everything running in tip top shape. The blower, whacker, and pressure washer are winterized, but the tractor has been having some trouble running lately. The Kohler CV25S engine will start fine but after a few seconds it will die. If I pull the choke out for a minute I can get it to stay running though. I did a cylinder leak down test, the first cylinder had excellent loss values, while the second cylinder was leaking air like a balloon with its end cut off. The spark plug was also covered in carbon
So with a leak down test that negative, and thinking I heard air coming from the oil pan, I decided the probable cause was weak piston rings letting the combustion gasses into the crank case.
The cylinder head had quite a bit of carbon build up I’d need to clean off. You can also notice on the right side of the head oil stains from where the head gasket was not sealing correctly against the engine block. A possible culprit to the starting issues! This would also mean I incorrectly diagnosed the issue as worn pistons rings. This is more likely where the issue is stemming from.
The sides of the piston and the rings appeared to be in great condition. No carbon buildup inside of the grooves, the oil control rings looked like they were operating correctly. After reading some more on a few tractor forums it seems the rings on the Kohler Commands rarely need their piston rings changed. I already had the piston out at this point, might as well re-ring it and deglaze the cylinder
The intake valve was in good condition. It made a proper seal. I cleaned off some of the carbon on the shaft but didn’t think any lapping would be required.
The exhaust valve was in worse condition. The bottom of the valve had something slimy on it that elbow grease and an ultrasonic cleaner was unable to remove. I’d have to take a wire brush to it in order to clean it off.
The seats were in good condition, no scratches or blemishes that could keep the valves from seating properly.
The head after a little bit of cleanup. You can see here where whatever was hitting the top of the piston also hit the bottom of the head.
I decided that since the engine was already disassembled I would go ahead and replace the head gasket on the other cylinder at the same time even though it isn’t having any issues right now. More experienced people that myself also recommended cleaning the valves as those are the normal culprits to poor performance with the Kohler Command engines.
* 2 24-841-04-S head gaskets (might as well replace the other one while I was at it)
* Cylinder honing tool to deglaze the cylinder
* Fuel pump and filter
* 24-108-05-S piston ring set
* 2qt of 10w-30 oil and a 52-050-02-S filter
* 6 ft fuel line
to complete the engine winterization process.
6 feet ended up being near exactly the mount of fuel line I needed to replace every piece of fuel lines from the tank to the carburetor.
After I received the parts I got to work deglazing the cylinders and getting the new piston rings installed
Got everything back together, and accidentally broke the oil pump! Ordered a new one off of ebay, waited a week, and got everything running again!
After taking it out for a spin I realized the engine was surging pretty bad:
I took the carburetor apart again to look for any remnants of dirt or gunk in jets or passageways.
Unable to find anything wrong, I decided to purchase a cheap knock-off replacement carburetor from Amazon to rule fuel issues out of the equation.
After receiving the new carburetor I found I was still experiencing the same issues. The only other troubleshooting steps I could think of would be the coils. The Kohler Command twin engines were manufactured with “smart spark” modules that adjusted the spark timing to improve engine performance. Kohler released a parts bulletin back in 2013 stating the entire spark system was being obsoleted for a different version of coils.
I found other videos on youtube stating similar symptoms, so I decided to replace yet another part, the Kohler 25-707-03-S kit.
Unfortunately, after replacing the entire ignition system and flywheel the problem persisted. After another leakdown test the intake valve on the number one cylinder was still leaking.
It was time to turn to the professionals.
I took the cylinder head to a local small engine shop to get it properly cleaned. Unfortunately again, not even the pros could do it correctly, after reassembling the engine the problem still persisted.