I have a briggs&stratton 500e 140cc push mower. It has always been pretty reliable for me, though admittedly I've never done any real maintenance to it other than add some oil here and there and I bought it way back in 2017, so yeah.

It crapped out on me a couple weeks ago in the middle of cutting and wouldn't crank back up. As I'm not real engine savvy, I tried adding some oil and accidently over-filled it by a lot, as it overflowed the fill port. I syphoned oil back out and tried to get it going again, but still to no avail. At this point I decided to figure out how to give it a tune-up, to the best of my ability at least. I took the carb apart and got it looking brand new, replaced the air filter, replaced the spark plug, cleaned out the gas tank, and completely changed the oil.

After all of this, it fired right up first pull and I let it run for 10 minutes or so before putting it back in storage until i needed it again.

Fast forward to last weekend and I'm ready to mow the yard with it for the first time since the tune-up. It ran for about an hour before it crapped out on me again. In that hour it was running, it seemed somewhat weaker than it'd been previously, meaning it was bogging down and getting close to shutting off in grass, though long and thick, that it used to power through no problem. This caused me to have to continuously back off and slow down to keep it from shutting off.

Upon further inspection after it crapped out, I checked the new spark plug and found it wet with oil. There was also some excess oil soaked into the air filter as well(coming out of the engine air intake line).

I cleaned the spark plug off and it ran for another 10 minutes and quit again. Just like before, I found the spark plug wet with oil and more oil had come out of the intake line and soaked into the air filter. I cleaned the spark plug off again, but this time I couldn't get it to start back up.

What do y'all think? I figured any excess oil from the over-fill would've burned off after it running for over an hour. Could I have over-filled it again when I changed the oil? When checking the level with the dip-stick, do you just dip it and check, or do you thread it all the way down, unscrew it and then check? I mean there's a difference of probably almost half an inch with those two options. I just dipped it and it read within range, but if I were to screw it all the way down and then check it, it WOULD currently be over-filled again.

If I did indeed check the oil level correctly and it's adequate as of now, could I still be dealing with the excess oil having gotten into places it shouldn't have when I over-filled it the first time, and that's what's causing me issues? Even after having run it for at least an hour after fixing the oil level? Also any idea as to why it seems to be running weaker/wanting to bog down and shut off easier?

Thanks so much for reading all this and I greatly appreciate any insight y'all can provide.

If you have a dip stick, put it in the tube, all the way. Not just at the top.  Level should be between 
the high and low mark.  
 
I believe you have the plastic carb.  The orifices in the picture shown, must be clear.   
 
Once that's clear, let the engine run.  It'll burn off excess oil in the muffler, etc..  
 
You should be good after that.
 
 

  

Yes that is correct, it is the plastic carb that I have and the part pictured. I cleaned everything completely and verified that the orifices pictured were clear when I took it all apart a couple weeks ago.

I just now went to take a look at the mower again. I verified with the dip stick screwed all the way down that I have the correct amount of oil. I pulled the spark plug out and it was wet with oil. I cleaned it off and re-installed. After this I tried to start it. Mind you, it's now been sitting for a week and this is the first time I've tried to start it since it quit on me last weekend. It fired right up with a plume of white smoke, which I assume is oil burning off. But it only ran for about 30 seconds before it shut back off. I pulled the spark plug and again it was wet with oil. So it seems like that is the issue? Excess oil still in the combustion chamber rendering the spark plug unable to spark? I cleaned the plug off and tried to start the mower again, but it would not start again.

So everytime I clean and re-install the spark plug, it comes back out wet with oil. Am I able to stick a rolled up paper towel or q tip into the combustion chamber to try to clean up the excess oil or is that a no-no?

If your oil level is correct and oil keeps loading up in the combustion chamber,
you very likely have a bad piston oil ring. 
 
That would keep oil from getting up into the chamber..  
 
Does the dip stick screw in (like a full turn) or just about 1/4 turn (when set in the tube)?
 
If the oil level is correct, I would hold the throttle open more than 3,600 RPMs (within reason)
and try to blow out the excess. 
 
If it doesn't clear, the internals, (rings) would need replacing (if the cylinder is ok and not worn out). 
 
.
 

SRTsFZ6
If your oil level is correct and oil keeps loading up in the combustion chamber,
you very likely have a bad piston oil ring. 
 
That would keep oil from getting up into the chamber..  
 
Does the dip stick screw in (like a full turn) or just about 1/4 turn (when set in the tube)?
 
If the oil level is correct, I would hold the throttle open more than 3,600 RPMs (within reason)
and try to blow out the excess. 
 
If it doesn't clear, the internals, (rings) would need replacing (if the cylinder is ok and not worn out). 
 
.
 

Someone else on another forum said it's not a dip stick, but a screw stick. It's threaded(4 or 5 threads, a few turns) as it doubles as the cap for where you add the oil.

If it's a "screw in stick",  wipe the stick dry, poke in the hole, DON'T TWIST IN, and pull it out quickly
(so it doesn't wick up oil).
 
That'll be the correct level...
 
.