The following article excerpts are reprinted with permission. Original article credit goes to Direct Parts / CCP by Direct Parts.
Carburetor tuning basics:
in mind that motorcycle carburetion is a compromise and is never
perfect. The Basic circuits are the Enrichener circuit:
Aids in cold starting and warm-up. Accelerator Pump: Aids
in starting and off idle transition. Idle & Low Speed circuit:
Controls idle, and helps transition to Midrange. Midrange circuit:
Controls steady throttle and light acceleration. High speed
circuit: controls hard acceleration and wide-open throttle. All
circuits overlap, but each one has a primary function. Idle (throttle
closed): controlled by slow speed jet and mixture screw. Midrange
(steady throttle and light acceleration): controlled by needle. High
Speed (Wide Open Throttle): controlled by main jet. Tuning: Base Line
Tune each circuit for it's primary function. Idle & Low Speed circuit,
control 100% of idle and about 25% of right off idle low speed. The
primary function of this circuit is the Idle. There is nothing else
controlling fuel here. Wide-open throttle hard acceleration, when the
slide/needle is all the way up the main jet is your primary controller
of the fuel. The needle controls fuel for everything in-between. The
needle starts to fuel right off idle with light throttle. As soon as you
start to open the throttle air velocity increases under the slide, and
fuel drawn from around the needle, even before the slide lifts.
Bottom line, set the idle jet and fuel mixture screw for the best idle.
Set main jet for best wide-open throttle. The needle will control mid-range, steady
(cruise) and light throttle.
Carburetor tuning using the Plug Chop method
commonly asked question about carburetor and Fuel Injection is "How do I
know if my jetting is correct? A typical question might go; " I just
added a new motorcycle exhaust system to my bike. Do I need to change
the jetting? My exhaust color seems to be fine." Or, "I don't have
access to a Dyno, how do I tell if my jetting is correct?" Long before
Dynamometers became common, we used to do jetting using the "plug chop"
method. We will discuss it and some general jetting rules here. From the
factory, your motorcycle came with standardized EPA mandated lean
jetting and restrictive intake and exhaust. In some locales, this
jetting is so lean it must be changed to prevent damage to the engine.
The standard jetting has varied over the models of carburetors and model
years. All of the motorcycle part information below is based on your
engine intake system being properly sealed and no other problems exist.
An intake leak will throw spark plug readings way off. One of the most
common problems is the off idle "pop" and backfire at slow engine
speeds. This is usually caused by the idle mixture being excessively
lean and is easily corrected by correct jetting and / or the use of a
jet kit. The ignition timing needs to be correct and the ignition system
must be functioning correctly. A weak or poor spark will drastically
alter your plug readings. We like to do our jetting in two stages.
First, we need to get the idle and low speed jetting correct and then we
work on the high speed or main jetting. In our example, we will discuss
the Keihin CV (CVK40, CV40) used since 1989 on Sportsters and 1990 on
Big Twins. The adjustments may differ for S&S "E", HSR Mikuni and others
such as the Quick Silver from Edelbrock. Before you start, have a
collection of fresh unused plugs gapped and ready to use. We will need
at least three new sets for this test. We use Champion plugs for plug
chops. You can get them at local motorcycle parts stores for far less
than the OEM plugs. EVO Big Twin use RN12YC. EVO XL and TC88 use the
RA8HC. On the CV, adjust the idle mixture while idling. Closing the idle
mixture screw slowly should cause the idle to become rough. Slowly turn
the screw out until the engine again idles smoothly. Add approximately
1/8 to ¼ turn more. If closing the screw makes no difference in idle
speed or smoothness, you will have to use the next smaller pilot jet.
Until the 2000 model year, most Big Twins have a #42 as the stock pilot
jet. 2000 models use a #45 stock. In some cases, depending on the bike
setup and altitude, a #45 may actually be too large. An aftermarket #44
pilot jet works very well under most conditions. If you have to turn the
screw out more than 3 turns, consider increasing jet size to the next
larger pilot. The JCP
EZ-Just idle needle for Keihin CV 40 (CV40, CVK40, CVK44, CVK34) comes in handy here as it allows you to adjust
the mixture without tools.
Note: On older Keihin CV's and all Keihin Butterfly carburetors check the inlet fuel elbow for cracks or leaks. The injection molded plastic elbow breaks easily and can cause a dangerous fuel leak. If your elbow is cracked I suggest that you play it safe and replace it with a Hi-Flow brass fuel inlet elbow for all Keihin CV and Butterfly carburetors. The H-Flow elbow fits the CVK34, CVK40, CVK44 Keihin series carburetors.