Simplicity Legacy w/25hp Kohler Command running really rich

macindude · Sep 20, 2023

EDIT: Posted resolution at the end. One cylinder wasn't developing much power due to a bent intake valve pushrod and a bunch of unburned fuel was getting through that side. New cylinder head kit fixed it.

I have a Simplicity Legacy (non-XL) with the Kohler 25hp gas engine. While I was mowing a couple of weeks ago, I fairly suddenly lost power. I found I was able to start the engine, but it didn't have enough power to engage the PTO for the mower. I was able to limp it to my garage.

It appears to be running REALLY rich. Sooty exhaust, carbon fouled plugs, has more power when the air cleaner is off, etc.

I have already taken the carb apart and cleaned the heck out of it, including the main jet that they helpfully hide underneath the fuel shutoff solenoid. I spent way too much money on a carb rebuild kit, could have just bought two cheap Amazon carbs.

At the moment, it really won't run at all unless I remove the air cleaner, and the choke needs to be *off* for it to start. Choke plate in the carb is wide open.

Any ideas as to a good troubleshooting strategy to fix this? I'm not a pro mechanic, I know enough to be dangerous and I like to fix my stuff.

RandyT · Sep 20, 2023

Pull the dipstick and make sure it doesn't smell like gas. The gas fumes in the oil can cause the same effect as the choke being on and will sometimes get better with the air cleaner off. the other thought is the float valve isn't sealing causing overflow of the carb leading to a rich flooding situation.

tomplum · Sep 20, 2023

Other notes: If this has the accelerator pump on the carb, that could create a rich condition if damaged. As well as the fuel pump itself letting fuel into the oil. You seem to be describing a rich condition, but have you confirmed that both cylinders are running? An easy way if you have an infrared thermometer is to compare the exhaust pipe temps.

macindude · Sep 21, 2023

Update:

Upon suggestion from @tomplum, I pulled out my temp gun to verify that both cylinders were running. I was sure they were, since it was just as smooth (or not) as normal, but... They weren't. The right-hand cylinder (the one on the ignition module side, opposite the fuel pump side) exhaust is not even hitting 100ºF in the first minute or so of operation.

I had already replaced and gapped the plug, but I pulled it anyway and it was making what looked to me like a good spark. Dunno what the "proper" way of testing this is, but I just pulled the plug, plugged it back in, started the engine and held it against one of the cylinder head bolts for a ground. It started sparking before I even made contact from the central electrode through the ground and to the bolt on the engine, maybe 3/4" inch away.

So... What else would cause one cylinder to run fine, and the other cylinder to not run at all?

RandyT said:
Pull the dipstick and make sure it doesn't smell like gas. The gas fumes in the oil can cause the same effect as the choke being on and will sometimes get better with the air cleaner off. the other thought is the float valve isn't sealing causing overflow of the carb leading to a rich flooding situation.

It actually does kinda smell like gas inside the filler. I'm guessing this is just because of how much I've been running it with one cylinder dead?

tomplum said:
Other notes: If this has the accelerator pump on the carb, that could create a rich condition if damaged. As well as the fuel pump itself letting fuel into the oil. You seem to be describing a rich condition, but have you confirmed that both cylinders are running? An easy way if you have an infrared thermometer is to compare the exhaust pipe temps.

It does have the accelerator pump, but when I cleaned up the carb I did take that apart and inspect it, and it's in surprisingly good shape. But you were right, only one cylinder is running as described above. What could be causing that?

RandyT · Sep 21, 2023

What model of Kohler engine is this. Different issues can arise depending on model.

5030 · Sep 21, 2023

I know in my case. it was mouse chewed wiring that went from the magneto to the coil packs. Just enough insulation chewed off to render the one cylinder inoperative with the air shield (top of the motor installed).

Topzide · Sep 21, 2023

Check the compression on the cylinder not coming up to temp. A build up of debris around the cylinder head can cause head gasket leak. Having spark is only part of the equation. You need spark, fuel and compression to run. Have also seen the decompression Valves stick open.

tomplum · Sep 21, 2023

Checking the compression is a good thing. Commands are mechanically pretty stout, but a migrating exhaust valve seat happens in a blue moon. Usually a non firing cylinder on a Command goes back to an ignition issue. It sounds like you have SAM ignition. Like Randy said, the model and spec is real helpful in situations like these.

When you say soot, I'm thinking loose black practically powdery stuff as opposed to hard carbon and plug fowling type of thing? Correct?

So an engine that decidedly runs on 1 cylinder leaves unspent fuel in a cylinder more so with compression, but it would figure that it would run rich anyways as twins are like that. That would be soot.

I hated buying accelerator pump kits, so cheers to a good diaphragm. The carb kits as you eluded to are crazy expensive, let alone to price a carb complete.

Grab a compression gauge off the Napa loaner counter and do a comparison of the two cylinders or a leak down test if you can swing it.

macindude · Sep 21, 2023

RandyT said:
What model of Kohler engine is this. Different issues can arise depending on model.

CH25S - 68583.

Topzide said:
Check the compression on the cylinder not coming up to temp. A build up of debris around the cylinder head can cause head gasket leak. Having spark is only part of the equation. You need spark, fuel and compression to run. Have also seen the decompression Valves stick open.

I don't have a compression tester... At least, not yet.

But, just to ensure that there isn't anything wrong with the plug, I'm going to swap the spark plugs tomorrow and see whether the problem follows it or not.

The buildup of debris causing a leak is a definite possibility too. I did do a lot of cleanup when I had the engine out briefly, but I didn't pull the covers over the heads so there's still some in there.

If there's a valve sticking - What's the solution for that? Sounds like major surgery at that point.

macindude · Sep 21, 2023

tomplum said:
Checking the compression is a good thing. Commands are mechanically pretty stout, but a migrating exhaust valve seat happens in a blue moon. Usually a non firing cylinder on a Command goes back to an ignition issue. It sounds like you have SAM ignition. Like Randy said, the model and spec is real helpful in situations like these.

When you say soot, I'm thinking loose black practically powdery stuff as opposed to hard carbon and plug fowling type of thing? Correct?

So an engine that decidedly runs on 1 cylinder leaves unspent fuel in a cylinder more so with compression, but it would figure that it would run rich anyways as twins are like that. That would be soot.

I hated buying accelerator pump kits, so cheers to a good diaphragm. The carb kits as you eluded to are crazy expensive, let alone to price a carb complete.

Grab a compression gauge off the Napa loaner counter and do a comparison of the two cylinders or a leak down test if you can swing it.

Here's the data plate:

Didn't know there was such a thing as loaner tools at Napa!

RandyT · Sep 22, 2023

This is an Analog Spark Advance Module engine. So the issue could be with the one of the two ignition modules, or the ASAM itself. Either way I would do the 24 755 308-A ASAM to MDI conversion.

The other thing is the CH25 engines had issues with blown head gaskets. On the Command twins there is no decompression system but wouldn't be bad idea to pull the valve covers and make sure the push rods are in place and one valve isn't sitting higher than the other.

MossRoad · Sep 22, 2023

I have a Kohler command CH25. I had a similar experience. In my case (may not be yours), on the cylinder that's not firing, the nut that holds the rocker arm came loose, the rocker spun sideways on it's post, and that let the pushrod come out from under it. So the valves on that rocker would not function. I was lucky, in that all I had to do was put the pushrod back under the rocker arm, put the arm back in position, and re-torque the nut.

So it might be worthwhile to pull the valve cover on the side that's not working.

In my case, it was the side with the oil filler cap, so I looked in there and that's how I saw it.

Of course, one of the valve cover bolts snapped off when I was removing the cover, but that's another story.

Good luck in your troubleshooting.

California · Sep 25, 2023

5030 said:
mouse chewed wiring that went from the magneto to the coil packs. Just enough insulation chewed off to render the one cylinder inoperative with the air shield ... installed.

Reading that brought back a nightmare. 70's 400 ci Wagoneer. The vacuum advance caused the condenser wire to move, and ground against something to fire at the wrong time during starting and under hard acceleration. Easy fix ... ... ... after I found the problem.

The pro mechanic (well, gas station employee) I bought it from hadn't figured it out, so I bought it cheap. We both thought for a while that the car needed an exorcism.

macindude · Sep 26, 2023

Welp... This is the intake pushrod from the #1 cylinder:

Now, to figure out what caused that!

tomplum · Sep 26, 2023

Nobody stuck a Briggs and Stratton sticker on it, did they?

You got this? At least you have a good side to compare things like guide positioning, how far the valves are protruding from the head which helps to determine seat and guide health, how free the valves cycle etc. How many hours and are you good about maintenance?

MossRoad · Sep 27, 2023

macindude said:
Welp... This is the intake pushrod from the #1 cylinder:

View attachment 823420

Now, to figure out what caused that!

As I mentioned before, is the nut loose on the rocker arm for that?
Mine actually pushed a hole through the composite valve cover.

macindude · Sep 28, 2023

tomplum said:
Nobody stuck a Briggs and Stratton sticker on it, did they? You got this? At least you have a good side to compare things like guide positioning, how far the valves are protruding from the head which helps to determine seat and guide health, how free the valves cycle etc. How many hours and are you good about maintenance?

I'd have to look at the hours. It's well past 1000 hours.

As far as good at maintenance, I've been working on lawn tractors that belonged to my dad and then to me since I was little (49 now), and I took a 1-week course in small engine repair once upon a time. However, I'm clearly not doing it very much, because I still have all my 10mm sockets and wrenches.

MossRoad said:
As I mentioned before, is the nut loose on the rocker arm for that?
Mine actually pushed a hole through the composite valve cover.

They may have both been loose - As soon as I got the valve cover off, both pushrods and rocker arms were loose (rods fell right out of their spots in the arms).

Got a suggestion on Facebook that maybe a guide moved due to heat. I'm not sure how I'd be able to tell. I'm going to get the whole cylinder head kit for #1, and probably at least do a new head gasket on #2 and torque everything properly while I've got it apart.