http://www.performancejet.com/ttjetboatenginecooling.html

Read that it should answer everything you need. Easiest fix is to install a hand ball valve before the t fitting and adjust the handle opening to when the engine hits around 1800 200 rpms.

Are you testing this at the lake or with a garden hose running to the engine?

I have the same problem. At idle water still flows even with the Bassett T. I just went to Home Depot and bought the brass 1/4 throw 1/2" ball valve (same as what is installed at the jet). I now have control on water flow to headers right from my seat. I will run the headers dry before I shut off from now on. About a minute or so, not at all concerned about blueing the headers. I'm going to have them ceramic coated black later anyway.

I spent $30 at Home Depot and $12 on heater hose.