Issue with Blower Motor Not Spinning Up
OK, so that is a "Split Phase" AC induction motor, meaning there is a centrifugally operated switch inside of it that is supposed to change state at a certain speed in order to allow the motor to run. Without getting too deep in the weeds of AC motor theory, single phase motors will not rotate on their own, they need to be "tricked" into starting to rotate, then once they are, they will continue doing so. One of the "tricks" is called a "Split Phase" design. The winding is in two parts, one of them designed to"bump" the motor in one direction, the other designed to run it once it begins rotating. The first part has low power capability, all it is supposed to do it get it started (bump it), then a centrifugally operated switch on the shaft kicks in the other part of the winding that actually creates the rated torque. That centrifugal switch is not functioning correctly in your motor, so it appears as though your start winding is being left in place and the run winding is not being connected.

These switches wear out, or they get filled with dust and gunk over the years, especially if the blower is being used for dust collection or extraction, like in a cooking vent hood. In most cases, that switch is located under the end bell housing on the NON drive-end of the motor, meaning the end WITHOUT the shaft that is connected to your clutch. The simplest attempt for you to try is to open that up and spray in a bunch of "Electrical Contact Cleaner" that you can buy from most hardware or auto-parts stores. Just do NOT spray it in there with the motor plugged in, and let it thoroughly dry out before you do apply power; that cleaner is flammable. 

In this photo, the centrifugal switch is not labeled, but is directly below the word "winding":


If that doesn't work, the contact points are likely burned and it needs to be replaced, you can buy replacement centrifugal switches over the internet from numerous sources, probably even Grainger (who sells the Dayton line). That's not a job for the feint of heart however, there are springs and levers and wires that must be carefully disassembled and reassembled. So if you DON'T want to tackle that, you can remove the motor and take it to a motor shop, they can do that for you. Or, you can just buy a new motor, it might cost you less...