Where to set the riser depends on weather and how you will using the tractor.
In cold weather, or for continuous light loads set it to the "hot" position - ie turned to direct exhaust around the intake. Otherwise set it to "cold".
You get a bit less power with it set "hot". And under continuous heavy load in warm weather the engine may have a tendency to knock. But in cool, damp weather the engine will warm up faster and generally run better with the riser set on "hot".