Thanks for the info, guys. I saw the instructions on how to renew the seat for the ball check and I did do this procedure. I thought maybe the seat had eroded somehow, although it looked OK so that is why I went with the little needle valve.
I hadn't thought of the air horn gasket affecting the vent. I will verify that the vent is indeed open. Right after the oem carby started messing up a couple weeks ago, I had the engine running with the air cleaner off. I noticed that there appeared to be a little gas splashing up into the vent, so I thought there must be something wrong with the needle and seat. I tested the old needle and seat by turning the air horn unside down and attemped to blow into the fuel inlet. The needle valve held, but I thought there must still be something wrong with it so I installed a new needle and seat w/ gasket and checked the float level. Subsequent times I took off the top after running the engine I noted the float bowl was about 3/4 full of gas, so I assumed the float level was correct.
This is the only YF I have had any trouble with and I am running 3 of them on different vehicles.
