I know you said you have a good throttle servo, but are you using nyrod? If so the heat from the mufflers will expand it and it will cause what you are having. Solution... use a long metal threaded rod attached to the end of the nyrod that runs through the hot area. Better yet, replace the nyrod with Sullivan Precision Rod and do the metal trick also... this will never expand/contract.

Just wanted to start with the easy stuff before you take the carb apart.

Cheers