The only thing you needed to do was to soak it in lacquer thinner and then blow it out.
It would have been like new!

I think an autopsy on the muffler is due. It would make a nice follow up video. Thanks again

Thank you!! This was my issue also.

My 2 cents for what it is worth. Retired now but worked on small engines for 55 years. I would need more information from the owner. Here are my thoughts on how the muffler got plugged. First I would ask if mixed fuel was being used. Was the generator left on Eco mode and idled for long periods of time. Were any fuel additives used. What weight motor oil was being used. Also for what it is worth after determining that the muffler was plugged I would have removed the spark arrestor and left the muffler attaching bolts slightly loose. If the engine was able to start even though it was not running well I would slowly start to tighten the mounting bolts with the possibility that it may clear itself.

Great diagnosis, don’t think I would have figured this out

I think I might have tried submerging the plugged muffler in some cleaner or another like Gunk or some such for a few days to see if that might unplug it.  Also, I'm really like to see the old cut open to see what's plugging it.  Around here I'd be suspect of mud dauber wasps.  Good video as always Buck.

Yep that’s a expensive machine put the right parts in it, great job!

Thank you for the video

I would never buy a new muffler, especially because it carboned up. Got to be a way to acetone the whole muffler. Not to pour in acetone but to soak the whole thing. I would never consider buying a new muffler, that seems ridiculous.

Good Catch! I wonder what was plugging the muffler?...Maybe a mouse,or wasps?..I would be curious

How many hours on that generator that the guy say?    I have the same unit. That’s a BIG muffler, looks like a fairly easy install though.  Good catch thanks for the video.

Wow… Mine had the same symptoms, it would run poorly for a second or two and die.  I had already tried cleaning then changing out the carb, cleaning the fuel screen, pulling the CO2 sensor wire, oil sensor, etc..  I thought it might have a bad intake valve as it was spitting out of the carb a bit but compression and leak down tests looked fine.  I finally came across your video but mine only had five hours on it so I was dubious.  With nothing to loose I pulled the muffler.  It started on the first pull!!  WTF?  The muffler seemed clear, I blew it out from the exhaust backwards with compressed air and couldn’t find any blockage or debris.  The spark arrester screen seemed clear just a little red with light rust.  I pulled out the arrester screen and reinstalled the muffler without it for kicks.  It started right up and ran perfectly.  I still can’t figure out what this was about but it’s running fine.  I just ordered a new spark arrestor screen off Amazon for $20.  Thanks for posting this, I was really getting frustrated and would have never have thought to check for a blocked muffler!

So Buck - Just what would cause the engine to run so rich that soot would deposit inside the muffler??? Well, how about the owner forgetting to turn the choke off after it started, time after time?  Running with the choke on (you mentioned he ran it a lot) CERTAINLY would do that over time.  
To start, I would have used compressed air from the engine side, using a rag to stop any blow back, until just the air would no longer blow soot through and then switched to a GOOD degreaser like LA Awesome.  Shaking it a lot until it was cleaned out and then flushed it with a garden hose should have finished the job.
After draining and reassembly, a final "Test Drive" for 10 minutes would have completely dried everything out inside the muffler. 😀

Nice video, but what plugged the muffler? The unit wasn't used much to cause exhaust to clog. Is the right fuel being used or the right oil?? Was the spark plug fouled... Too many unknowns and the same thing could happen soon after this repair. Iagree with the other comment of cutting the old one open, but try to soak in a cleaner before wasting a usable part...

Could you not use a propane torch and burn the carbon out?

Very unusual reason for not running.

So you paid so much? or the price was chip For the thig

Should’ve soaked the old muffler in gasoline for 24 hours

Thanks for the vid.. great diagnosis.  But, look at those fingernails.. are  you a guitar player?

Open the old muffler!!