Stalling Problem?

[Randyjaco]

The bike is a Verucci 150. It has been running OK, but is now seriously stalling under throttle. It starts easily and and engine runs smoothly, but the max speed is only 26mph. When I twist the throttle it just boggs down.
I checked the spark plug (@50 miles old). It's clean, but dark (running rich). The carb seems to be getting plenty of gas. I checked wiring for tighness and arcing.
I don't see anything wrong.
What am I missing?

TIA,

Randy