You may need to adjust the settings on your carb, like the throttle pin setting. There are four notches on the pin that goes through the carb piston. The lower the setting, the more gas you get at low end, and vice versa. What size carb are you using? If its a 15mm, you may want to go up to a 72. If its a 14, a 64 might be okay, if its a 12, it think 54 is a good bet. I'm trying to help, but Bing is not my forte. Also make sure your piston is in the right direction.
A biturbo should give you carbon trouble, normally, after only 300 miles.
Carbon buildup was a good guess, puch love to carbon up. The generic symptom of clogged exhaust is poor throttle response, in addition to quiet and slow running.