From my reply to a similar question:
Well, I looked it up in the service manual last night and the adjustment procedure runs 3 pages with tables and illustrations; its fairly involved. Briefly:
Using the described tool (Belt Tension Gauge PV-43532) to apply 10 lbs. perpendicular pressure to a cool, dry belt.
Chieftain - 1.3" (34 mm) or 11 notches up on belt guard
Vintage/Classic - 1.1" (28 mm) or 8 notches up on belt guard
You should check the belt at 4 different points 90 degrees apart rotating the the wheel clockwise as viewed from the belt side of the motorcycle, placing a mark on the wheel at the tightest point to use as a reference. Rotate the wheel clockwise 1-2 revolutions until the reference mark is lined up with the tension setting window in the lower belt guard. Use the tension gauge to apply 10 lbs. of pressure against the belt at the center and count the notches it deflects in the window. Adjust to above specs.
Now, you must also check the run-out of the belt on the rear pulley turning the wheel both forward and backward, which involves adjusting the left-side axle adjuster so that the belt tracks in the center of the pulley or you will experience chirping and rapid belt wear.
--- Randall