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.

YS Performance
1295-C Industrial Ct.
Gardnerville, NV  89410

Phone:
775-265-7523 (Information)
775-782-4562 (Parts & Service)

FAX:
775-265-7522 (Information)
775-783-8518 (Parts & Service)

Email:
perfspec@775.net

Ron,

I actually had that same screw fall out of my motor a few seasons ago at
the top of a loop and just one week before the Nats. Dirty undies - ugly
but successful auto. I found on subsequent engines that the screw was
never in very tight, and that problem was made worse by the plastic
washer under the screw not allowing for good torquing. Yamada has never
been one to tighten screws really well on their motors (complete contrast
to the gorillas at OS!).

Anyhow, after that screw came out, I found that a 4 mm set screw would go
up in that hole and bottom out just fine. I used a tiny bit of red
Loctite with it. Apparently Richard thought this was OK, because when I
had the motor in for service, he didn't mess with the screw. This is all
moot on the 80, thank goodness. For those customers who want to get in
the air now, either a real short 4mm bolt or pan/cheese head screw or the
4mm set screw that I used works fine to plug the hole.

FWIW,

Ben