Yamaha XS400, Mikuni BS34: What's this hole in my carb? Followup!

So as a followup to this same thread last year (http://tinyurl.com/
3afnzq), I hit it again this spring. I soon realized my carb holders
had massive vacuum leaks and at least one float bowl was overflowing
raw gasoline into my air filter. Two new intake gaskets and two new
needles and seats later, it appears to be running correctly. At first
I was befuddled when it did nothing but afterfire when I tried to
start it with the electric starter, but apparently my battery was weak
because it fired right up the first time I kickstarted it. Finally, it
no longer hangs up at 3-4000 RPM and there is only a tiny amount of
master idle speed screwed in with 3.0 turns of the idle mixture
screws. No backfiring either.

Now I need to correct all the other little things and see what happens
when I go for a ride...

Thanks again guys for all the patient advice last year. And that
mystery hole? Definitely blocked with a brass plug. Must be a leftover
from creating one of the other passages during the manufacturing
process.

 

Ok, I'm soooo close.

After a few runs around the block, I got some backfiring on the right
side, as in I could feel the blast on my leg coming out of the airbox.
Now it seems to run well with 3.0 turns on the left and 4.0 turns on
the right. I'm assuming I should *not* leave it like this and instead
synchronize the carbs with a set of vacuum gauges before doing
anything else. Right?