Sputtering at High Speed, TBR7

[Bamakojon]

I have a Tao Tao TBR7 that I'd like a little help troubleshooting. It has a Mikuni VM26 carburetor and an aftermarket exhaust--other than that it's unmodified. It starts and idles great, but it has a problem with sputtering when holding high speed (45+ mph) when it gets hot. It also feels like it has a delay in throttle response at lower speed, especially when rolling on or off, like it's "catching up" suddenly to the new throttle position.

I bought it used with the aftermarket parts on it, so I figured that maybe it was running rich because the previous owner put an oversized main jet on it, but I pulled the carb and the main jet is a 110, which from what I've read shouldn't cause issues. I'm going to be replacing the air filter because it is filthy, so hopefully it breathes a little easier, but I don't think that's the root cause. I did change the spark plug from the stock chinese one to an NGK and it seemed to help, but the problem didn't fully go away.