your new YS61ST2 idles fine and hovers fine, but when you go to full
throttle, it goes lean, check to be sure a screw isn't missing from the
carb. We will check all the engines we ship from now on, but if you're
missing the screw you must contact YS Performance at the address below or
check out Ben Minor's solution
If the screw is there, check to see that you're getting good tank pressure. If not, check your tank and fuel lines to make sure there are no leaks. If all that checks out, the next thing to do is to replace the check valve. Still no joy...be sure your muffler is on tight and, if it's a two piece Hatori, make sure the pressure plug screw hasn't come out of the stub header. I know there's no pressure, but if that screw is missing it will cause the engine to run faster, thus leaner.
If that doesn't do it, I would suggest calling YS to see if they have some other things to check.
I actually had that same screw fall out of
my motor a few seasons ago at
Anyhow, after that screw came out, I found
that a 4 mm set screw would go