Could be as simple as not moving air effectively when spinning the wrong way, so no load (or small load) on motor. Spin the right way, air is moved, motor slows down, work is done. Try blocking off the intake or output and see if the lack of air movement causes the fan (& motor) to speed up again.
There should be a schematic printed on a sticker on the motor. Without knowing the make and model, or looking at the diagram. I'm guessing based on other motors.
One common "standard" is as follows:
- White = grounded (neutral) (clockwise).
- Black = High speed.
- Blue = Medium speed (medium low).
- Red = Low speed.
- Brown = Run capacitor.
- Brown w/ white = Run capacitor.
- Green = Ground.
- Yellow = Medium speed.
- Purple = Grounded (neutral) (anticlockwise).
Optional wiring available on some motors.
However, without the make and model, or the schematic, this is just a guess.
Update
The two brown wires (Br and Br/Wh) should be connected to the capacitor. If you don't have one, you may need to install one. The brown wire from the old motor should have been connected to it, so try to remember where you removed the brown wire from.
The white wire from the motor, should connect to the white wire in the junction box. This is L2/neutral, and completes the circuit.
The remaining wires are for selecting the motor speed (Blk, Blue, Red), so you're going to have to figure out which speed you should be using. If you can remember which color wire was connected to the red wire in the junction box, that should be the same color wire to hook back up. Typically the blower is run at low speed when in heating mode, but that's no guarantee that your equipment was set up that way.
I wasn't able to locate any documentation on your equipment, and without a schematic I can't be any more specific. If you could include a good photo of the schematic, I could show you where to make the connections. You should be able to find a schematic in the manufacturer's documentation, or on the furnaces access panel.
Best Answer
Yes, your diagram is correct.
Your new motor appears to not be reversible so hopefully it spins in the correct direction.
Colours on equipment wiring generally do not mean anything aside from what is printed on the diagram; there are no consistent standards or requirements.