On my STX, flushing with the hose connected to the factory fitting creates an air lock and prevents flow thru the head and a good flush (I run in the bay and need a good flush). This weekend I cut into the cooling line from the pump and flushed it that way and it worked great, head stayed cool and I was able to flush long enough to do some good. I will install fittings that will make it easy to flush this way from now on. I had cleaned all the water jacket cooling passages when I had the top end apart is why I am sure it is an air lock issue.