Somewhere along the line, STs in the 40-45 range came over with carb. intakes that are just a bit too big. At one time ST supplied a restrictor to insert in the carb and that made it much more easy to handle. I have some with and some without.

If you have trouble with the transition from idle to full throttle, the restrictor may be the answer. Just did a 40 and changed a @#*%XX into a sweet machine. Take a piece of tubing, brass or alum., that fits into the carb. Remove the spray bar assembly and the idle needle. drill a hole to fit spray bar through the sides of the tubing. Insert tubing then reassemble the carb. You may lose a few RPMs but it will stop the transition trouble as the high-speed will not be so rich that it will flood the engine in the early transition.
In addition, I suggest you don't try to run the engine with the muffler all the way in on the pipe unless you cut off the pipe a bit.