Really sounds like it might be a main jet issue, because it really only seems to happen when you are near or at full throttle. What's odd though is if you didn't change anything and it did this just after sitting. I would say if you had upsized your main jet and then this started happening, it could be that you went too large and the engine is just getting flooded with too much fuel and bogging down at full throttle. You said you checked it for corrosion, because that might be one other issue if it was sitting for a long time with regular gas in it instead of ethanol-free gas. The ethanol over time can attract moisture from the air and cause corrosion of some internal parts. What I can't really tell from your video is weather it seems to be running too lean or too rich when the throttle is wide-open. That would make a difference as to what to do to correct it.
Yeah, that's odd. You didn't change anything with the air filter or exhaust, did you? Sounds like you didn't. Did you check the slide needle on the diaphragm, the one that opens the main jet? Some of those are adjustable with a little C clip at the top. Maybe that clip came off or got adjusted to the wrong slot and the needle is not pulling all the way out of the main jet at full throttle.
I have 2 gy6 buggies, 150cc and 170cc, and am having slightly similar problems, but more so with the 170cc. Bought it stock from a friend with only 32 miles on it, so basically brand-new. First thing I did (after the usual CVT upgrades) was put a high flow flow exhaust on it and got rid of the air box and put on a Uni filter. I believe the stock main jet that came on it was a 114, so I went to a 128 that came with my filter.
The problem I'm having is that it accelerates just fine, but I get a lot of exhaust popping on deceleration and it will not drop immediately back to idle and takes a few seconds to return to idle. I'm fairly new to this stuff, but from what I've read that would indicate either an exhaust leak or a lean condition on the idle circuit. I went from a 32 to 38 on the pilot jet thinking that might help. I checked the exhaust and sealed any areas where it could potentially be leaking, and it doesn't appear to have one. I also checked for vacuum leaks by spraying carb cleaner all around the intake manifold and carburetor.
One other thing to note that is probably important is that on both of my buggies, I can turn the idle mixture screw all the way in until it is seated and it doesn't cut off the engine. In fact, it probably idles slightly better. Any more than one or two turns out and it starts to bog down and sounds really rich. So I turned it back to where it idles slightly on the rich side, which is only about 1 turn out, but it is still having the exhaust popping and slow return to idle problem.
I kind of reached the extent of my knowledge on this, and I have a jet kit with multiple different sizes of main jets and pilot jets, so any suggestions would be helpful. I did check both mixture screws to make sure they weren't broken off at the tip, and they both look fine. I should also note that on the brand new buggy, the carb was sealed from the factory with Welch plugs on both the bowl and the mixture screw. Needless to say I drilled those out. What was interesting to note was that the new buggy from the factory was set with the mixture screw at 4 full turns out. At this point I'm wondering if an adjustment to the main jet might help, though it wouldn't make sense to me, or if maybe the float level needs adjusting.