OK. I'll do that.

I tried greasing the oring, but it didn't make a difference. 

I removed the silencer from the muffler, and it helped a little. They I tried loosening the exhaust pipe at the engine. Had a lot more power, and would rev to 6000rpm before it would stumble. So it looks that is at least part of the problem. (My apologies to the neighbor for all the noise without the muffler.)
So I have a few questions yet that I hope someone can answer:
1.  Redline on the tach is 7500. Should it rev that high? I would think so, but it doesn't want to run above 6000.
2.  What are my options for cleaning the exhaust? I have heard that the carbon deposits can be burned out, but I don't have a torch big enough for the job, and I can't build a fire outside (burning ban due to extreme fire danger and 100 degree heat). Is there some way to chemically clean it out that works?
Thanks for any suggestions.

Your options are limited if you can't burn it out. Look down the tail end where you removed the end muffler & see if there's an obvious restrictive build up visible & poke it out & shake it out. I put my end muffler in the enclosed log fire & does a good job. Any engineering shops around that could get away with smoking things up. Have you checked the cylinder port--exhaust port. Unlikely, but small motors ie small ports build up around the port edge & have seen little yamaha 80 YG1T almost blocked off after years of farm use.--as i say, unlikely but!!. Burn the tail muffler out too if you go to an engineering shop.

I had the cylinder bored and cleaned it all up already, so no restriction in the exhaust port. I can see some carbon buildup inside both pieces of the exhaust, but nothing looks restricted. Still, I need to get it cleaned out somehow.
Thanks.

Did you put the piston in with the arrow on top towards the front of the engine (exhaust port) and the windows in the piston skirt on the intake side?

Yeah, piston is installed correctly. Appreciate all the suggestions. I'm missing something, just not sure what else to check.
Going to try and get the exhaust system cleaned out next.

Going back to the carb O ring you've already replaced .
Did you replace with the original Yamaha part or have you just sourced the O ring elsewhere ?

Oring is important for sure--but--Listening to the video, Just get that pipe sorted & report back. Might end up having to cut it open but unfortunate that no fires allowed to burn it out complete. Think you answered the question by releasing the pipe at the front & doubling the revs. Winter here for us. Stay safe.

Got the oring from Yamaha dealer.