Low Speed Carb
The fuel mixture screws are indeed located behind the
brass plugs. The brass plugs must be removed to access the screws (GO
Idle Adjustment Screw Brass Plugs ON THE
PAGE ). Be *very* careful about
drilling out the plugs-use a drill stop. If you hit the screws with
the drill bit, even for a moment, you risk running the screw down
hard against its seat, destroying the screw and possibly the carb
The fuel mixture
screws deal w/ mixture at idle and a little bit above idle. They
will determine off idle throttle response and extremely low idle
cruising. Turn screws in to lean the mixture out, turn them out to
richen the mixture.
Here are some
rough guidelines to determine if you are running rich or lean at
various rpm's. Examining the plugs is really the best way to get
accurate results, but these guidelines will help you shoot for a
direction (leaner or richer).
Idle and off
Poor off idle throttle response, idle that flutters a little after a
throttle blip then slowly returns to the idle that you've set,
popping back through the carbs when the throttle is blipped.
Off idle response may be good unless plugs are fouled from a way too
rich mixture. After a throttle blip the motor will dive down below
set idle and will either die if way too rich or may return to set
idle if it can recover.
To adjust, run
the motor until warm and idle is stable. Turn each screw in, one at
a time, until the engine stumbles, then back the screw out until
just rich of peak idle (idle drops again). Then turn them back in
to peak idle. Repeat for each carb. If your screws are out more
than 3 turns, the jet probably needs to be swapped to the next
larger. The final position should be around 2.5 turns out on a
stock set-up bike.
screw adjustment vs. Carb sync
The pilot screw
adjustment and the carb sync are really different adjustments all
together. If you had never done a carb sync it was probably good
advise to do it once before fooling with the mixture/pilot screws ..
or anything else. The carb sync is intended to equalize the opening
of the carburetor throttle plates, (this is beneficial throughout
their range of operation).
The reason you
see different recommended mixture/pilot screw setting in the manual
for the different models is because these bikes all have different
size pilot air jets, (primary ... the one on the top of the carb ...
the secondary pilot air jet is the same on all models and it hides
behind the slide rubber inside the carb). Air from "both" of the
pilot air jets mix with gasoline from the "pilot fuel jet" to
produce the mixture that you control with the mixture/pilot screw,
(except when you open the throttle a little .. because there is a
bypass hole .. but that is another story).
you'll find the screws set differently in each carb is because at
the factory, (hopefully), they set these precisely using a CO meter
connected to each cylinder via the little 1/4 pipe plugs you see in
the factory exhaust pipes. These differences make up for the
uniqueness of each carburetor and cylinder it is attached to,
(rather unavoidable .. as minute as it might be). Cranking these
screws out beyond where they might have been set originally will
increase the CO readings for the respective cylinder, (make it
richer at idle). Even though there might be other ways to achieve
your objective ... this works pretty good for adding additional fuel
at off idle ... and at light, (no load ), up to about 1/8 throttle
throughout your mid-range, (which hopefully you will find
There is a limit
to this benefit and if you do more to your intake etc you may find
you need to do something more substantial for you light throttle
fuel requirements and do something different, (like make one of the
pilot air jets smaller or the pilot fuel jet larger etc). As most of
these bikes have the pilot screws set on the "lean" side for
emission purposes you will hopefully benefit to some extent by
backing them out a little. The models with the bigger primary pilot
air jets etc are going to require a little more twist to achieve a
comparable mixture result, (unless you are doing the ear routine or
have your own $125 JC Whitney CO meter).