Take a look at how your exhaust is directed where it exits from the muffler. If the exhaust is directed down toward the ground..........it can possibly be doing something similar to a flame type weed scorcher.
And if so, and you are making any "pauses", like if you need to stop and reverse direction of travel......that short pause as you stop and shift may be enough time for the exhaust gases to damage the grass blades.
Apparently this has become a "known issue" with several OEMs as those have started including language in their owner's manuals that cautions about the sort of damage you are reporting.

Thanks much. I'll check this asap after rain ceases. Do you happen to know any of the recommended fixes/remedies for this, if that is the culprit?

I'll also check the underside for any leaks, but that seems less likely because I haven't notice any at the place where it's parked.

Cheers,
Z.H.

With this being a "used machine" (but new to you) it might be that a part is missing, such as a deflector or even a section of the muffler outlet pipe..........that is allowing the exhaust to be aimed toward the lawn instead of its correct direction (as determined by the OEM).
It will make your task (of solving the problem) easier if you find that a replaceable part is missing.
On the other hand, if....this turns out to be a poorly designed detail of the machine, it would be necessary to fabricate a deflector plate, a baffle/diffuser of some sort, or some type of pipe elbow to redirect the exhaust stream.
And this type of modification to the machine cannot be taken lightly because a mistake made by you could might redirect the exhaust the wrong way causing damage to the tractor or physical harm to yourself (exposure to exhaust fumes or higher noise levels).

IF you have a "sacrificial" area of grass, park it there a few days without running and see if you get a BIG brown spot. That would mean a fuel leak.

@B.K.: I like the sacrificial area/parking it suggestion, and I had done that in the last few days and no fuel leak, thankfully.
@Mownie: I also have identified the old rusty muffler deflector that apparently fell off the mower in the days after I began using it and was hidden nesting in the grass. So Mownie was spot-on about this being the culprit. The muffler on my rider -- not the OEM one by the way, according to diagrams of my Briggs engine online -- does in fact need a new deflector, or I'll maybe swap out the whole muffler. Fortunately, the seller has offered to help me deal with this. Thanks guys.

I had that same problem, but i found that the carb bowl gasket had a small drip in it, and, if i parked it in the grass, it would only drip a wee bit, but i got the browned grass thingie.

It's good that the seller is willing to help out. Without some sort of expressed warranty strings attached to the sale, these things generally fall into the "condition as is" group.

Oh, and I meant to add that if you are looking at a muffler in a Briggs engine IPL..............that muffler is most often found/used where the engine is in an open or exposed venue. Because of tractor cosmetics and other considerations you will often find that the tractor OEM, not the engine OEM, is responsible for the muffler you find on the chassis.

Ditto Mownie's comments... and should be able to download a parts list for your Yard Machine tractor to confirm muffler p/n by lifting up the tilting seat, getting the full model # and Serial # off the ID plate located in that cavity - then go to the link below to get a download..

The parts list should clarify as to who to order from.

Here is a link that might be useful: MTD manuals

{{gwi:334674}}
I get the same thing but I think it is related to this little guy.

wheely boy. We have one just like yours. Ours is so excitable and "hyper" that we have to dress him out with a waist band diaper to prevent unwanted spots anywhere.
Your guy must be even more frenetic than ours as I noted the full body suit. :^)

He is our first small dog and first male dog. I'm not sure if it is size or sex that drives his behavior there is a good reason why he is wearing prison garb.

Ours has such a problem case of separation anxiety that as soon as he realizes he has been "left behind" he just about panics. Never before saw a dog quite like him.

My wifes pomeranian has a bad case of sep anxiety as well she takes him everywhere now little devil started peeing everywhere when left alone and chewing on everything too.... Damn ankle biter!!!!

I can tell from that picture---you are on his S-list! Beware, the left alone Pomeranian! Better treat it nice from now on!

Yeah he dont like me or having his picture taken he bares his teeth and snaps at me every time i git near him. I've been bitten 3 times in the last year.

He's bitten you three times??? why is he still alive?

If that critter lived here--he'd be taking a hike over the Rainbow Bridge!
A flu shot, way back in 1960, made me highly allergic to dogs! It has not worn off. Going into a home that keeps a resident dog, sends me into breathing problems in about 15 minutes!

Zach, I'm wondering if you might have an infestation of chinch bugs rather than a lawn mower problem. If the brown spots started out looking like they just needed a good watering, but progressed to - uh - DEAD, I would do a chinch bug test.

I have a new John Deere S130 riding mower, I get the same circles after mowing.