Sputters when outside temperature is hot, runs fine in the cold

[Dom]

I have a well used 50cc Chinese Bronc Rally and have maintained it myself for the last 5 years. This spring I got a new battery, cleaned the carb, replaced the drive belt, and replaced a rusty clutch. It ran like new.

A few months ago it began to sputter a high speeds and has been getting slowly worse, top speed of only 25-30. This only happens when the outside temperature is above 60F or so. It runs fine in colder weather, like in the morning, and always starts fine in all conditions. I can ride it to work fine in the morning 5 miles away but the ride home is awful and I slow down traffic.

I replaced the spark plug, coil, and bystarter (autochoke) with almost no improvement. The gas tank seems to be vented properly. I have no idea what could be causing this to happen. My current best guess would be something funny with the exhaust that acts up when it's hot outside.

Any ideas?