Black smoke only while engine cold

[TexasMowerMan]

I have a troybilt 42" riding mower with. Kohler engine. The mower is old and I have replaced the carb on it about 4 times.

This year, I decided to try my hand at soaking and cleaning the gummed up carb instead of buying a new one. I was able to get the engine to start very reliably, like a new carb.

But, now the engine emits black smoke when the engine is cold. A few details...
1. The choke works fine.
2. The smoke is not coming out of the muffler.
3. I think the black smoke is coming from liquid that is on top of the exhaust manifold. And the smoking stops once the liquid finishes burning off.
4. The exhaust manifold has been covered with a nasty black residue for years, i scraped it off this year, now it smokes more. I have a feeling that the black residue is the same stuff causing the black smoke, and by scraping it off, now the liquid heats up faster, with the centimeter of black residue previously, it allowed the liquid to do a slow burn and less noticable burn off of black smoke.
5. No smoke after about 5 minutes of running, but the engine does smell like burnt hydrocarbons the entire time i mow.
6. Smoke only emits from the exhaust manifold area (front of engine), nowhere else.

I have fixed about 5 mowers in my life, but I have never worked on the actual combustion part of the engine or the exhaust. So, assume I am a newbie when giving advice.

 

[bertsmobile1]

Sounds like a leaking exhaust gasket or blown head gasket

 

[TexasMowerMan]

Sounds like a leaking exhaust gasket or blown head gasket

Would a leaking exhaust gasket cause black smoke? Or would it simply result in smoke emission before going through the muffler?

Also, how hard is it to replace the exhaust gasket and head gasket? I am kind of interested in doing the work as a learning experience. I have never disassembled an engine before.

 

[TexasMowerMan]

@bertsmobile1 after stewing on things for a while, I watched some video of people replacing the gaskets on Kohler engines. Seemed simple enough, even more simple on a Kohler Courage (which is what I have). Then, I went out and took a closer look at the liquid on top of the muffler (took some pics too).

Looks like my valve cover is leaking... which makes sense, the oil in the valve cover sits and puddles in the bottom of the valve cover after I finish mowing. Then spends a day or two leaking onto the top of the muffler while the mower sits idle. Then, when I go to start it up, the muffler gets hot real fast and starts burning off the oil.

One thing that threw me off was that the internet was saying that burnt oil would be white... but, I think that is 'burnt clean oil', my mower is overdue for an oil change, so that is probably why my smoke is black.

Anyway, I also see a tiny leak at the head gasket too, although it is probably not bad enough that it needs to be fixed. I am going to go ahead and replace all of the gaskets so I can get some experience doing it, should be fun.

Main Leak onto muffler from valve cover


Discolored spot is a potential sign of a leak from head gasket, although it could just be spillage from the valve cover leak.

 

[TexasMowerMan]

A couple more things so that this thread is hopefully as helpful as possible to other readers.

Here is the gasket kit I purchased from Amazon:

Amazon.com: Kohler 20 841 01-s Cylinder Head Gasket Kit, Natural : Patio, Lawn & Garden

Amazon.com: Kohler 20 841 01-s Cylinder Head Gasket Kit, Natural : Patio, Lawn & Garden

www.amazon.com

$21.25 and free shipping at the time of posting. Weird thing is that the gasket kit part number listed on Kohler's website is '20 755 05-S', and that exact part number is on Amazon for about $50. But, if you search by the head gasket part number ('20 841 01-S'), you get almost the same kit for only $21.25 (which is the one I purchased).

The two videos I watched which gave me a good idea of how much work was involved with replacing the head gasket were these two... (I went to youtube and searched for Kohler Engine Head Gasket Replacement):

Taryl fixes all video, funny dude, knowledgeable, not the perfect video because it is for a different model kohler engine, but he does a good job explaining everything along the way, which I found helps reduce my anxiety before doing the work.

Same engine as mine, but on a different device, kinda boring video, but it was helpful to see the work done on the same engine as mine.

 

[bertsmobile1]

Now that we know it is a courage engine, check the front 4 bolts on the top cover
If they are loose then look for a crack running down the crank case from the cylinder / case corner on the exhaust side.

 

[TexasMowerMan]

Will do. I took off the top cover to replace the magneto recently, i didnt notice that any looseness, but I will definitely check for a crack next time I am out there.

 

[bertsmobile1]

Different covers.
If you have the bucket engine where the sump is on the top it is called a "cover plate "
not the blower housing .
These courage engines have the oil filter at the top ot the engine and pointing down so you can change them without getting oil everywhere
Down here this engine was only fitted to Husqvarna's but your mowr came out of the same factory (AYP ) so you could have the same engine .
Search for Kohler crack and you will find out all about it.

 

[TexasMowerMan]

I figure I will post an update, since I was on the site for another reason.

All my parts are in, I plan on doing the work soon. I also googled 'kohler courage crack' to find what you were referring too. I checked my engine and didn't see any cracks (whew!), but there might be one I couldn't see. When I get around to pulling everything off to replace the gaskets, I will check for cracks more thoroughly, but my gut feeling is that mine is not cracked, as the 'kohler crack' stuff I read made it seem like the crack would be pretty obvious.

 

[Scrubcadet10]

I thank my lucky stars everyday, the cub cadet has the courage 19 bucket engine in it... So far none of the common problems, 10 years and 270 hours. except for the leaky needle, the motor has been bullet proof.
I make it a practice to check all the bolts with a wrench at the beginning of each mowing season when i tune it up.
Tex, here's a video on the valve cover gasket common problem