Fresh water flush clogged?

[Rational]

I have a 2004 F225TXRC. The last fresh water flush using the attachment on the motor gave me very little water. The hose has full pressure. I squeezed the flushing water line as far as I could reach (boat on a lift, I did not want to go swimming). Assuming it was blocked somewhere, I removed the hose connection to see if the running engine would blow it out. It did not, I had only a trickle of water. Apparently it is clogged or there is a one way valve that has failed. It did seem to flush properly using the muffs on the lower end.
I'm concerned about running the engine if the blockage is inside the block, although the pressure relief seems to work OK when the engine is running.
Any suggestions on where to look?

[boats.net]

Probably need to remove ignition coil bracket and check hose fitting for blockage.