Just went and checked my three way valve (still factory original ) and it does have a spring in it .i stuck a small gauge wire in one of the outlets and you can feel the spring pressure against the valve
I cant see how it could work if it didnt have one otherwise it would be continuosly bleeding back to the tank ,
From what ive worked out is that it only works when the
Engine is at idle speed the fuel bowl is full so fuel is bypassed through the valve back into tank to lessen the risk of flooding ,and at high rpm the valve is closed to lessen the risk of leaning off and holeing a piston
This is of course all irrelevant if the rest of your fuel system is not set up properly
Eg.. Float level and condition of float valve,fuel pump ,fuel pump pulse line etc...
Hope this helps
Ps i have also tried the 190 maint jet mod in the fuel return line and that also works
Maybe hoser can add to this he's probably had more to do with this issue than most of us
What do you think hoser