I can shut off the mixture control completely

Okay what mixture adjustment are you referring to here? The High end, Low end or the regulator on bottom of crank shaft? I have the FZ version but I'm sure they adjust identical. I pulled all my needles out before high low and regulator and got it going again. It just took a little longer than a normal 4 stroke. I can try to help out.

Oh also look at your check valve and make sure its going right way and it is not leaking. Also make sure you have it on the correct fuel line.

Also may want to get some advise from Dave in the YS engine support section. He knows YS and can get you going very quickly.