Diagnosing problem on Kohler V-twin (Exmark mower)

[lawncareguy]

Hi all-
I've been trying to figure out the problem with a Kohler CH23S 23 hp engine on an Exmark mower. It starts and runs, but stalls when trying to cut grass- seems to have no power. I checked the spark plugs and they seem fine- visual inspection is good, and I can see them spark if I hook them up to a weed whacker (can't crank engine now because I have taken it apart, but plug on the suspected bad side was wet when I took it out). When I first started working on the mower, I tested it by pulling the spark plug wires one at a time to see which cylinder the engine was running on- I found that it would run on the right spark plug wire alone, but not the left one alone. Since the spark plugs were fine, I concluded that the problem was the ignition coil, and I disassembled the front of the engine and took it out. HOWEVER, I measured the resistance of the old coil between the spark plug boot and the screw holes in the coil and found the resistance to be 9.4 k-ohm, which is very similar to the resistance of a new coil I bought, and seems to be in the acceptable range. The ignition coil has one wire going to it, which is the one that grounds it out to shut off the engine. I tested this with the key on off and run, and both positions seem to be working properly. I can't quite remember what the air gap measured before I took the coil out, but I did check it and must have thought at the time that the air gap was reasonable, so it must have been somewhere between 11 and 16 thousandths, likely on the low-medium side of this range.

So my question is this: Could the ignition coil be the problem, and if not, what else should I check?

I have been using this manual, and the electrical systems part starts on pg 41

 

[RandyT]

Could you get a compression reading for both cylinders. It may be a valve problem, fuel problem, or something similar. But to narrow things down compression would help.

 

[CalG]

Poor High voltage insulation within the coil is difficult to measure with a VOM.

Any breakdown can have a significant effect in output at the plug gap.

 

[lawncareguy]

Thanks for the advice.

I may try a compression test soon, but I will have to get some of the loose covers out of the way first. I have the fan cover loose right now, but there is a shaft on the flywheel side, so it is difficult to get the stuff out of the way since I am working on it in the mower.

If it had a fuel problem, wouldn't I see similar performance on both sides? I mentioned in the original post that it will run with the spark plug wire on the right side by itself, but not on the left side by itself.

Engine has just under 400 hours, not sure if that is useful info, but thought I should mention.

 

[CalG]

Yes, fuel issues would show on BOTH sides.

You must have either a problem with the spark (change plugs!) or a mechanical issue (valves etc)

 

[lawncareguy]

CalG, are you saying that a bad ignition coil could still have good resistance measurements? I just replaced a coil in a John Deere, and that coil was completely open circuit. This one in the exmark/kohler seems to have readings not much different from a new part

 

[CalG]

yes!

If the windings are intact and not broken, but the insulation is weak, the voltage developed will "jump the gap", and so output a "weak" spark.

This is by far the most common failure for a high tension coil. Insulation break down! I've got a pail of then in the shed!

 

[lawncareguy]

Thanks for the response, CalG. I will probably check the compression by hand-rotating the engine with the plug on the good side removed and feeling whether or not there is any resistance to the rotation on the bad side's compression stroke. If there is at least some compression, I will probably replace the coil as planned. The owner seems competent enough to be doing regular maintenance, so an internal problem is less likely at 392 hours. I will try to update you guys when I figure it out if I can remember.

 

[RandyT]

CalG, are you saying that a bad ignition coil could still have good resistance measurements? I just replaced a coil in a John Deere, and that coil was completely open circuit. This one in the exmark/kohler seems to have readings not much different from a new part

As a Kohler dealer, I have a special tester to test the high energy coils used on Kohler command engines. On that engine you can have a coil that will jump the gap on the plug with the plug grounded on the side of the engine, and still have a bad coil. I am thinking something compression related could be the issue though.

 

[lawncareguy]

Conclusion: ignition coil was bad despite its relatively normal resistance measurement.

Before I put the new coil on, I did a simple compression test by taking the spark plug out on the good side and just hitting the key very briefly to crank the engine about 90 degrees on the compression stroke of the problem side. The flywheel rebounded, indicating I had some level of compression. I replaced the coil on that side and now have power on that cylinder.

Thank you to all who contributed to this thread.