Until last year I never had issues on muffs. Run (Mercs/JohnRudes) until the thermostat opens and consider the flush complete. That said my new to me Yami 130 2 stroke (first yami for me) will not run on muffs. It runs, a few min pass, and the overheat alarm goes off. After replacing 100% of all cooling related parts the solution was....you need to run it in a barrel
. For some reason on muffs the bypass wont open allowing water to heads. The one upside to this is my neighbors and I learned how much more quite running in a barrel is
. This year I make a dedicated flush tank.