G5200 garden tractor with 600cc 3-cylinder
The owners-manual suggests regular cooling-system flush-n-clean using Kubota cooling-system cleaner.
1) drained coolant from radiator and engine, remove lower hose.
2) used garden-hose to flush both engine and radiator thoroughly.
3) Replace lower hose, filled with water and radiator-cleaner.
4) Mowed leaves for several hours.
5) Drained (lots of black crud came out)
6) Fill with distilled water, run for 10 minutes, allow to cool, drain.
-- Repeated rinse 4 times
7) Filled with 50/50 mix
Interesting note: For each 'rinse cycle', after the drains stopped flowing, I used leaf-blower into radiator-opening and got a lot more water out of the drains.