Specifications

Start by dialing in the mains. For one mod on the 38s like filters or an exhaust change, I
would go up 1 size, maybe 2 at the most. For the 34s, it's been reported that 1 size up
improves a totally stock bike, so with any mods, I would start at least 2 sizes up. Test
your jet change by running the bike through the midrange, say from about 3K, up to
near redline. You must use absolute full throttle for at least some of your test runs or the
glitches may not show up. This is because of the vacuum lifted slide. It will mask over
minor faults if run easy. Do some easy or normal wind-up tests as well, in fact, try them
first. Major jetting flaws will show with them but the minor ones will only show, as I said,
under full throttle applications. If the bike pulls clean through this RPM range with no flat
spots (lean) or no stumbling or break-up (rich), go up a main jet size. If you do
encounter a giant flat spot where the bike just kinda dies, then takes off, that's lean and
you could probably jump up 2 sizes. Keep increasing the mains until you start getting
break-up or stumbling in the upper midrange, say from 4 to 5K. This indicates your
larger main is bleeding over and over-richening the upper midrange. This usually occurs
before break-up in the main circuit itself, like up near redline. Break-up near redline is a
definate indication you've gone too large on your main. You're seeking the size that runs
clean AND pulls the hardest. Time to move to the midrange circuit .....
As your larger main bleeds over into the upper midrange, it will make it too rich.
Compensate for this by lowering the needle (raise it's clip) in the slide. With the BS34s,
you're kinda stuck here as their needle is fixed and not adjustable. The good news is
that they were set up so lean from the factory that the bleed-over from larger mains may
not cause a stumble. It may just make the mixture right. If you do incur a stumble in the
upper midrange on the 34s and feel you need that large of a main, your only option
would be to purchase the adjustable Canadian needles that MikesXS now offers. Once
you eliminate the upper midrange stumbling through needle adjustment, you can try
another size up on the mains. This usually brings the stumbling right back, though. You
can drop the needle again, but I'm not a big fan of running needles in their lowest (or
highest) positions. The carb makers choose a needle that can be run in it's middle
positions. This locates it's tapered portion best in the needle jet for fuel metering. The
other "gotcha" with lowering (leaning) the needle is that, while curing the upper
stumbles, it makes the rest of the midrange rather flat and unresponsive. For this
reason, I like to lower my needle the least amount possible, even resorting to shimming
with a small washer to acheive 1/2 step positions. Many times, when lowering the
needle, you will create a flat spot just off idle, during the idle circuit to midrange circuit
transition. Time to move on to the idle circuit .....
If you've created a flat spot just off idle by lowering your needle, you'll need to increase
the pilot jet size. Any pilot jet change needs to be followed by a re-adjustment of your
mix screw. You've just richened the idle fuel mix so the mix screw usually needs to be
turned in (leaned) slightly to compensate. The 34s also have an air jet you can change.
This feeds air to the pilot jet to create the fuel mix. Along with increasing the pilot jet
size, you can decrease this air jet to richen your idle mix on the 34s. The 38s have an
air jet as well, but it's fixed and can't be changed, so it doesn't enter the tuning equation
with them. One size up on the pilots is the norm but you may need more. Re-set your
mix screws and test for the flat spot off idle. In 2nd or 3rd gear, slow down to near