Since this is a flat head engine (Briggs 122K02-0623-E1) about the only other possibility is a bad breather assembly.

Briggs part# 590395

About $5-6

OR- a hole in the piston!

Did you add 20oz of oil- or possibly more? If you check the oil level- does it seem high or possibly gassy smelling? If those are in check, a breather is probably a good investment as Bill says. You will also want to know that the o ring is present in the cap of the dipstick and no other obvious oil leaks.

But none of this was happening until after he worked on it. So what could he have done to precipitate this?

I checked the oil level after doing the oil change and the dipstick read that it was full, but not over full. After the problem started the first time, I checked the level again, still good. Then, while scratching my head in frustration, I checked a third time, this time I pulled the stick, wiped it, dipped it, checked it, wiped it, dipped it, checked it again and wiped it. Still good. So I feel fairly confident the oil level is correct (though I will probably check again tomorrow morning, because I simply can not give up on things like this until I know the problem...)

I will order a breather assembly and swap that out. If that doesn't fix the issue, I will start saving for a new mower. And probably tear the entire engine apart until I find the actual problem, just to put my mind at ease.

Even though I don't know small engines, I will probably lie awake tonight running through my head the things I do know about mechanical repair, trying to figure it out.

"But none of this was happening until after he worked on it. So what could he have done to precipitate this?" That is a bit unclear as there was a rough running problem. Had the filter previously been soiled? It seems as though you were diligent in checking the oil level. But does 20oz sound correct?
Breathers by their design can turn into little oil pumps. Is it possible that tipping from the front would allow oil to go were it didn't belong? Possibly. Me, I would remove the air filter element and start the mower in a clean area and run until warm. If no oil starts pumping out, filter it up and try again in the grass. And do be sure the little ring seal is in the cap of the dipstick.

"Me, I would remove the air filter element and start the mower in a clean area and run until warm. If no oil starts pumping out, filter it up and try again in the grass."

Yeah, I did that. The air filter was new, but after the first oil spill it was ruined, since it was a paper filter (I will be getting a new one today). So the test I described in the original post was done without the filter in place or even the filter cover. I wanted to see when the oil started coming out and where it was coming from.

I didn't tip it at the front, I lifted it from the front and supported it on stands, so rear wheels down, front wheels up. But only as high as I needed to get my arms and tools under it and then only for as long as needed to work. Once the old oil started draining, I lowered the front back to level ground over the drain pan. While the mower was empty of oil, I checked the blade. After putting the blade and oil plug back in, the mower was once again lowered to ground and I added the new oil which, as I just checked, is an 18 fl oz bottle. So if anything, it was 2 fl oz short. At no point after replacing the drain plug and blade was the mower lifted up again.

But I will be checking the O ring in the dip stick. That is one thing that I did not think to check.

This Morning's Test Run:

Checked the O-Ring, it is in place.

Cleaned the spark plug and plug well.

Cleaned oil residue from carb.

Cleaned oil from air filter housing.

Checked any hoses for cracks, holes or bad connections. The hose going from the air filter housing to... something is a little loose, but in place. No leaks at the hose itself. However, this is the hose that the oil is using to enter the filter housing.

Pulled the cord 6 times without the spark plug. Installed spark plug.

Cranked the mower. No go. Removed the spark plug and added a few drops of fresh gas to the plug well, replaced the spark plug.

Cranked the mower. Beautiful sounding motor. Perhaps a little low on RPMs.

Watched the air filter housing, with cover off, and waited. After five or six minutes the first drop of oil comes. Over the next few minutes, oil dripping increases and a new development presents itself. The oil is now pushing out sludge or debris as well.

This suggests to me that there is a clog somewhere, that somewhere there is old, burnt oil that is preventing the new oil from flowing properly. But that's just my limited understanding of mechanics and I don't pretend to be an expert.

I'm sure you can understand that some pressure is developing in the engine. (Provided the oil level is staying the the same and fuel isn't entering the oil.) This isn't common for this engine. Bill's suggestion that the breather could be defective is what you are hoping for. The reed in a breather acts as a kind of one way valve. If the breather gets allowed to fill w/oil -it can't really vent and acts like a small pump. Oil must also be allowed to drain out of the chamber. Certain types of breathers lend themselves to this more so than yours however. You could still remove and clean the breather to give it a try. An external crankcase leak can also create breather issues. You may want to verify that there aren't other leaks. This engine has kind of a crescent shaped breather cover that leaks at times on top. That part # is 697734 & along with the 590395 breather that Bill mentioned is still cheap. Should the issue still occur, it seems cylinder related. Best you can do is try.

So it's pretty much Breather or Bust. With proper tools, I could 'probably' break the engine down to cylinder level and rebuild it, heck I did it with an '83 Mazda once, but... yeah, I don't have all the calibration tools I would need to do that. Hmm... I really need to invest in a torque wrench...

Thank you for your expertise in this matter. It is greatly appreciated. I'd buy you guys a beer or two if I could. Thanks again.

Okay, new Breather is ordered as well as a new muffler, since the present one is quite rusted.

A quick update:

Just took off the current breather and cleaned it as best as I could, making sure it wasn't filled with oil. When I took it off, the valve well (?) had a small puddle of oil at the bottom. Not much, just about four or five drops worth. After I had everything cleaned, I put the breather back on, left the muffler off and started up the mower. I was watching the filter housing for oil and when it started leaking again, I shut it down. When I stood up and saw the other side, there was a spray of oil on the mower's deck just below the muffler, so apparently it blew it out. I then took the breather back off, figuring it was dead for sure. As soon as the bottom bolt was loose, about a tablespoon of oil leaked out of the valve well and the inside face of the breather was coated in oil.

As I said, a new breather and muffler are on their way, but I want to make sure that these are still signs that the breather is shot and not signs of something else. Thanks again.

Not really much else to go on there. How do you think the oil appeared on the other side of the engine?

Actually yeah, never mind. The oil being blown out the exhaust would likely be from when the filter was first in place and allowed some of the oil to enter the carb and other areas. What goes in must come out, I guess. Sorry.

Popped the valve cover off, figuring I would just go ahead and add a valve cover gasket to my order, since it probably won't be long before it goes anyways. There is a light sheen of oil around the piston. After wiping and wiping, the oil keeps coming back. The piston has some wiggle room as well, only about a millimeter, maybe less. It's been a while since I've worked inside an engine, and I know that components tend to expand as they get hot, but isn't the piston supposed to be pretty tight against the walls? If it is, then that suggests, to me, that either the piston ring needs to be replaced or the piston wall has been warped. If either case applies, I think this may have gone beyond my ability to fix at the moment without getting some more specialized tools? Is it worth it to take it to a mechanic to have it done on a 6-year old mower (really not that old compared to many still going strong) or should I start pinching my pennies?

It is hard to say. Really, the last time that you ran the engine, it was pumping oil into the cylinder- so there is bound to be excess. You have the parts coming, and yes- add a head gasket to the list. Patience and see how it goes. In answer to your question, if it doesn't come out of it after sealing the engine up and installing the breather- it would time for that new mower purchase from what I see here.

Do you know how to read the marks on your dip-stick? Or, do you just merrily pour oil into the engine!? Sounds, to me, like you over-filled the crank-case (where the oil goes) and there is now residual oil where there shouldn't be any.
Also, the piston should not be rubbing on the cylinder wall. Thats the job of the rings, to keep the compression in, and the piston not rubbing.
A man brought his expensive self-propelled, walk-behind, mower in to my place--said he changed the oil, and refilled it, but he couldn't get it started after that!
Well, no wonder! He filled the engine to the top of the dip-stick tube! Said he kept pouring it in until he could see the oil! WOO-HOO!! I showed him the marks on the dip-stick and showed him how to check the oil on his new mower. I never did get all of the oil out of the engine!

"Also, the piston should not be rubbing on the cylinder wall. Thats the job of the rings, to keep the compression in, and the piston not rubbing. "

Then why would they bother with a piston skirt and WASTE all that metal?

Yes, I know how to read the marks on my dipstick. It wasn't over-filled.

After replacing the Breather unit (and cylinder head gasket) the motor ran much better. No more oil was spilling from the Breather tube after letting it idle for about 10 minutes. So I put a new filter in place and went to mowing the lawn. After about 20 minutes my power started to die and I was having to re-mow certain areas, so I shut it down and waited for it to cool. Upon pulling my spark plug, oil spilled from the plug socket. Not surprisingly, my dipstick now reads less than full as well. I'm no expert, obviously, but I'm fairly sure the combustion chamber isn't supposed to be half full of oil. So my piston ring and probably piston well are shot. I'm going to keep the mower on hand. Maybe some day I will find a good machine shop and have the tools to break it down to get it bored out and replace the piston and piston ring, but I'm going to start saving for a new mower.

Once again, thanks for all the help. The Breather unit was faulty and it was a problem. It just seems it wasn't the only problem.

Valve Cover gasket, not head gasket.

What about the top breather cover? Was that resealed or at least verified that there was no leak?

There's no leak in that area.

Oil spilled from the spark plug hole? And it wasn't smoking like a steam engine while you were mowing? Is it possible that your cylinder was spilling out gasoline?

Take the plug out and let it sit overnight and see if gas isn't leaking through the carb into the cylinder. It's not an uncommon problem.

Oil spilled from the spark plug hole? And it wasn't smoking like a steam engine while you were mowing? Is it possible that your cylinder was spilling out gasoline?

Take the plug out and let it sit overnight and see if gas isn't leaking through the carb into the cylinder. It's not an uncommon problem.

As far as the machine shop working on your engine....only do it if you are curious and want to play, otherwise, it's not worth it.

It was smoking when I first started it, but that thinned out during the initial idle time, I figured it was residue being burned off. As it was starting to lose power, it was starting to smoke again, not quite steam locomotive, but noticeable. I didn't let it run long after that, figuring it would be prudent to just kill it right then and there.

I did let it sit over-night, because... well, I'm not mowing with it any more. Today, I was transplanting my mother's older, but running, Briggs & Stratton motor that was on a rusting non-mulching deck to my non-rusting mulching deck (I checked bolt patterns and measurements before disassembling the two). While doing this, I noticed oil was dripping from the air filter cage again, so I popped it off figuring it was coming from the breather tube again. But from the looks of it, it was coming from the carb and not from the breather tube. All the oil was on the front side of the filter (under the carb in-take) with very little on the back side (under the breather tube). The driveway the mower was sitting on over-night has a very slight pitch toward the road and the front of the mower was up-slope (if you stood on a well-lubed skateboard you would probably start rolling toward the street, but I didn't need to chock the wheels of the mower to keep it from rolling, though I did... just in case). That would have allowed gravity to pull the oil from the front of the filter to the back. But the majority of the oil was under the carb. If my understanding serves, gas goes from the carb into the combustion chamber, which already had oil in it, so I'm guessing what I found today was just some of the oil looking for the easiest way out, which turned out to be back through the carb.

So I'm not 100% sure where the oil came from, but I feel safe betting its from the piston, I suppose I could take the cylinder cover off again and take another look, just to sate my own curiosity... but I have consigned myself to the understanding that it is just dead (to me). Hence the motor/deck transplant. Now we just need to get her a mulching blade to go with her mulching deck.

Thanks again, guys.

I could probably start a new thread for this follow-up question... but... eh, why add more clutter?

Okay, so as I said, after doing some measurements I swapped the motor from my mother's rusting deck to my mulching deck. But there was one measurement that I missed. The length of the crankshaft. After the swap I checked to make sure that there was enough clearance above the blade so that it would not contact the deck, but I didn't check the distance from the blade to the bottom of the mower.

My mother mows her grass at about 1 1/2" (which I don't really agree with, but it's her yard) and I like to mow mine at about 3". The problem with the current her motor/my deck configuration is that at the deck's lowest point it is cutting at 3". In a sense, this would be fine for me. But after pushing the mower through 4"+ of growth today I find myself fairly whipped. I'm not one to shirk away from exercise, but that was just brutal.

I am looking at the crankshafts now and I need to get a caliper to make sure, but I think I can swap the blade mount from my motor to hers. That looks like it might extend the crankshaft enough to make the mower easier to push. But if they aren't the same diameter, are there any other options for extending the length of a crankshaft so that a blade cuts lower to the grass?

There are alot of blade adaptors out there to extend the length of the shaft. You just have to find the right one.