Purchased this mower from Lowe's this spring. It ran great for a while, then it started to spit and sputtered and belch black carbon from the exhaust. Took it to a local repair shop. They said that we need to run 93 octane in our mower as ethenol isn't good for. So they put in a carburator kit, changed the oil, new spark plug, new fuel, sharpened the blades. Brought it home, mowed 1/2 of the yard and it started doing it again. Took it back to the repair shop, the plug was black, so it got a new one. Brought it home mowed the front yard. 1/2 way through it started again. Took it back and they kept it. Said they would contact Briggs and Stratton and talk to them. Got the mower back and was told there were no recalls on it... But to not run it under bushes or hit the side of the mower as it could cause problems. Mowed the yard today and it's still doing it. So what's your best guess.

Take it to a shop that knows what there doing,
93 octane will do more harm then the ethenol.
Ethenol runs hot but high octane runs hotter, NOT GOOD .
Ethenol is only bad for 2 stroke engines.
Sounds like a intake valve problem to me.
Check air filter , the shop should have done this.
Make sure its running at proper RPMS at high speed 3150 to 3200.
What spark plug is in it ?
Luck

I'm not sure what spark plug is in it... at first it was the standard plug... when we took it back the next time, I am pretty sure the person told us that it was a plug that was intended for mowers that run hot.

This is a sealed engine, does that help with your diagnois?

Also what type of fuel should we run in it?

Thanks for your help so far.

Not sure what a sealed engine is.
Chances are it's a briggs overhead valve maybe a 6 1/2 horse.
Run just plain regular unlead .
Some people will disagree , but that's because they do not go to small engine updates every year.

Thanks for the information. We will take all of this information into consideration and see if it helps.