new problem or been there awhile?
Normally a stumble as soon as the pedal is moved is due to an accelerator pump issue.

Remove the air cleaner,
Block the choke fully,if cold
Bump the accelerator a small amount to release the fast ldle cam.
Look down the center of the carb and as soon as the throttle arm is moved you should see two squirts of fuel, one into each bore. If you don't you have found your flat spot.
Could be several causes but start with the above