This is an excellent cut away side view of the Keihin CVK carburetor to help illustrate the different circuits and parts of the carburetor.
A- Air inlet for lower slide chamber (M).
B- Air exit for lower slide chamber (M)
C- Hole on the bottom rear of the slide
D- Butterfly valve and approximate location of bleed hole drilled by many Hi-Performance CV rework shops
C- Hole on the bottom rear of the slide
D- Butterfly valve and approximate location of bleed hole drilled by many Hi-Performance CV rework shops
E- Idle circuit bleed hole
F- Idle screw (needle)
G- Pilot jet
F- Idle screw (needle)
G- Pilot jet
H- Accelerator pump nozzle
I- Needle and Needle jet
J- Slide diaphragm
K- Fuel inlet and needle seat
I- Needle and Needle jet
J- Slide diaphragm
K- Fuel inlet and needle seat
L- Main jet and jet holder
M- Lower slide chamber (pressure)
N- Upper slide chamber (vacuum)
O- Slide spring
M- Lower slide chamber (pressure)
N- Upper slide chamber (vacuum)
O- Slide spring
Notes: Most kits offer a lighter spring and a needle with a faster taper but I find they aren't necessary and just a few jets changed will get you were you want to be. I don't like to drill the slide,It can cause drivability issues in the colder weather.
When installing the main jet holder be careful not to invert the needle jet. Installing it upside down causes fuel to dribble out around the needle and causes rough idle, poor throttle response and mileage. If you install a carb kit and have these type of issues look at the needle jet first. You can also start the bike and have someone hold the throttle open enough for you to see the bottom the venturi where the needle enters the needle jet. If you see fuel dribbling out your needle jet is probably upside down. Remove the bowl and the main jet and holder and reverse the needle jet.
Most CV rework shops will ask you what engine combination you are running. When you have a high performance camshaft with a lot of overlap (both the intake and exhaust valves are open at the same time), high compression pistons etc. The engine idle becomes rougher requiring a high idle RPM in order to keep from loping too much. To increase the idle RPM the butterfly valve has to be opened further allowing more air and fuel flow. The problem is that when the butterfly valve is opened too far the off transfer ports are exposed causing hesitation and other problems. The solution is to drill a small hole near the bottom of the butterfly valve allowing more air flow without opening the butterfly valve further.When installing the main jet holder be careful not to invert the needle jet. Installing it upside down causes fuel to dribble out around the needle and causes rough idle, poor throttle response and mileage. If you install a carb kit and have these type of issues look at the needle jet first. You can also start the bike and have someone hold the throttle open enough for you to see the bottom the venturi where the needle enters the needle jet. If you see fuel dribbling out your needle jet is probably upside down. Remove the bowl and the main jet and holder and reverse the needle jet.
Here's an enlarged and cropped image in order to show the idle circuit a little better.
1- This is where the fuel from the idle circuit enters the carb throat. When fully closed, the tip of the idle screw can be easily seen in the throat.
2- If you look carefully you can see how the fuel is metered at the transfer port by the idle screw.
3- You can tell that the idle screw is not fully closed since the spring is not completely compressed. From experience I would say this screw is about 1 to 1.5 turns out from fully seated.
3- You can tell that the idle screw is not fully closed since the spring is not completely compressed. From experience I would say this screw is about 1 to 1.5 turns out from fully seated.
Metering is actually controlled by all three parts of the screw (see inset 4, 5, and 6).
The inset picture shows the three steps of the idle screw. 4 is the tip, 5 is short small diameter part immediately before the tip and 6 shows the main part of the shaft.
The tip meters the flow in conjunction the first step (5) depending on the number of turns out from fully seated. When the idle screw is fully seated the larger part of the screw (6) moves up into (2) and restricts the flow to the point that the engine should not idle at all. If the engine falls off but continues to run even when fully closed it is a sign that the pilot jet may be too large or the idle is set too high to compensate for an intake leak. As the screw is opened, more of the port is opened as the main part of the shaft (6) and the first step (5) expose more of the port relying on the tip (4) to meter the fuel.
We found that the effective range of adjustability was from about .5 to 4 turns out. At 4 turns out the first step (5) is fully out of the transfer port and the tip is clear of the throat. If you have the screw that far out I would go to the next larger pilot jet. I try to keep the adjustment range from 1.5 to 2 turns out.
Also at 4 turns out there is little or no tension on the spring (3) and the screw can and will fall out along with the spring, washer and o-ring.This will give you an idea of the adjustable jet needles and how to richen up the mixture.
Using the stock jet needle, you shim it up with 2 - #4 washers under the head to raise it up in the slide.
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