Take the spark plug out
Turn on the ignition
Rotate the crank slowly CCW
When you hear a spark make a mark on the hub and another straight across on the case
With the piston at TDC and the throttle wide open the two marks will not coincide
Measure the diameter of the hub at the point where the mark is, multiply by .244
The two marks should be that far apart to fire at 28 degrees BTDC
Adjust the link from the carb to the timing ring to get that measurement