Kohler CH25S backfires under load

Siddhi

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Hi all,
I have a toro with a Kohler CH25S. I bought it used about 9 years ago and it has 2025 hours on it. I had a problem with one of my plugs fouling. It ran like this for a couple of seasons. But other than the fouling plug it ran fine. I only use it to mow my lawn and about every 5 mows I would have to clean the plug and it would go back to working fine. This year I started having a bad oil leak from the other cylinder. It was coming from the head bolt holes. So I researched it and everyone said to change the head gaskets. So I bought a kit and replaced the head gaskets. While I was in there I removed the valves and cleaned them (very little carbon build up, but I figured while I am there might was well do it). I ground the valves with some grinding compound to make sure they have a good seat and I replaced the valve stem seals as well. I replaced the intake, exhaust, head and valve cover gaskets and put it back together. Now the motor cranks and starts fine. It has a slight miss at higher rpms, but does not back fire. Once I turn on the blades the motor lugs more than it use to and the motor backfires from the exhaust pretty violently.

So I pulled the plugs and did a compression and leak down test. compression is 165 and 175. the leak down test shows about 17% on both cylinders so this is well with in good. So I though maybe the carb needs to be cleaned. So I bought a rebuild kit (not much in the kit) and cleaned an rebuilt it. But that did not help. The plugs do look wet when I take them out and it seems to be fuel from what I can tell. I would lean towards electrical, but everything worked great up until I put the new gaskets on. I also thought that maybe I had the governor spring in the wrong place and that was putting too much load on the motor. But I have tried every hole and it changes nothing.

So basically I am at a loss and any help would be greatly appreciated.
 

ILENGINE

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Intermittent spark on one cylinder.
 

Siddhi

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How can I troubleshoot that problem?
 

ILENGINE

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Inline spark tester while running. Watch to see if it misfires under load. May not be definitive due to the high energy output of the CDI modules on the Command engines. Meaning you can have a bad module and still have spark. Could try testing with a spark plug with the side electrode removed connect it to the wire and ground to the engine while cranking to check for spark.
 

Siddhi

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Ok I bought an inline spark tester and both side fire without any misses. One thing I did work on is setting the governor correctly. After following the procedure it runs much better, but I still get a back fire under heavy load. It also seems to be backfiring from the carb now and not from the exhaust as it was previously. It ran good enough to mow the yard, but something is still not really right with it.
 
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