why would a demo saw idle fine and run wide open fine but will not go from idle to full speed when i give it ?

There should be 2 air/fuel mixture screws. 1 for low speed and 1 for high speed. Since it runs fine wide open, but it sounds like it doesn't transition to full throttle very well, I think the the low speed screw needs richened up.

It will idle OK with a lean low speed mix, but is too lean to got to full throttle smoothly. It depends on the brand of carb as to which direction to turn it, but go slowly one way or the other until it revs cleanly. No more than an 1/8 turn at a time.

must have a carburetor problem, low end screw needs to be adjusted