First of all, welcome to the site!
Will it allow you to push the throttle lever all the way forward or will it resist? Are you getting full range of motion in the lever but once it reaches a certain threshold it just doesn't do anything? Are you able to bring the lever back towards you (idle down) past
the notches, and does it bring the idle down?
First place I would investigate its the governor. Try adjusting the bumper screw (21) [just don't put it in too far] or speed adjusting screw (8). I'm not entirely familiar with the governor, but according to the diagram it looks like if either one of these is too far in/out it might prevent the throttle from advancing properly, of course this is speculation. But I would definitely investigate along those lines.
Find out what range of motion the governor speed change lever (9) will allow. Also, does the engine hunt at all, not able to keep a constant RPM, just goes up and down and up and down? I'm sure the others will chime in with better assistance, I'm just trying to narrow things down.