CP Instructions - Polini Com

      

CP Instructions

ASSEMBLY AND CALIBRATE THE POLINI CP CABURETOR

1. CHOICE OF THE CARBURETOR DIAMETER OR

VENTURI

The carburetor must be chosen according to the

characteristics of your engine. If your priority is to increase
the performance, the choice of the venturi diameter is
fundamental because it affects the resistance that the
aspiration system provides to the aspirate flow, in particular:

The larger the venturi diameter is and the lower the

pressure drop will be than those with a lower venturi
diameter and with equal characteristics with consequently
power increase.
The smaller the diameter is, the more the flow speed will
increase improving the power output.

2. FITTING AND PRECAUTIONS

The Polini CP carburetor has slightly different dimensions

than the more common carburetors. At the time of purchase
it is therefore necessary to consider what adjustments are
necessary for a proper assembly. The most important
adjustments are the flanges to fit the filter and the curves to
connect the throttle cable and choke. Once you have
assembled the carburetor it must not hit or get in touch
with other engine parts or bikes parts (except for the intake

http://www.polini.com/en/technical-area/polini-cp-carburetors/instructions/[16.10.2017 21:20:15]
CP Instructions - Polini Com

manifold or the filter to which must be connected). The

Polini CP carburetor is sold with standard calibration that
has been set-up during the tests on the most famous standard
motorcycles and scooter or equipped with power unit but
whose engines do not exceed 12.000 rpm. For different
settings see the proper section. If your engine exceeds
12.000 rpm and it is equipped with racing products we
advise you to carefully read paragraph 11
“COMPETITION”.

3. FUEL SYSTEM

Polini CP carburetor is of needle type.

The fuel arriving from the tank is held inside at a constant
level thanks to the floating needle mechanism. The fuel level
in the bowl that arrives to the jets during its working
remains constant and therefore so it is the gap that the fuel
must pass to get into the spray nozzle.

The fuel level in the bowl is therefore a calibration element

of the carburetor, since when it changes consequently the
signal on the carburetor intake nozzles changes and
therefore affects the mixture ratio.

The adjustment of the level in the bowl depends on the float

lever arm/needle and the weight of the float itself:

The floating needle lever arm is not adjustable in the POLINI

CP carburetor. It is calibrated during its assembly and it can
only be controlled. (See photos 1 to check the float
measure). By installing a heavier float, the result will be a
richer mixture; on the contrary, with a lighter float the
carburetion will be poorer. The floats are classified
according to their weight and they should be placed inside
the bowl according to precise provisions in order to achieve
the best working. If you use special fuels make sure that the
synthetic rubber at the ends of the needle is compatible with
it. The diameter of the needle housing is another important
element during the tests for the carburetor setting because it
fixes the fuel quantity and its speed when entering in the
tank, in particular:
• if the diameter is too small you risk to get a poor
carburation because the bowl may empty very fast than the

http://www.polini.com/en/technical-area/polini-cp-carburetors/instructions/[16.10.2017 21:20:15]
CP Instructions - Polini Com

needle can make the fuel pass through, with the serious risk
of damaging the engine.
• The diameter of the POLINI CP carburetor needle housing
has been studied to avoid this problem as the hole diameter
is oversized.

4. THROTTLE VALVE

It is an adjusting element that determines the area actually

available for the flow passage. It’s a cylindrical valve that
slides across the venturi in an appropriate housing obtained
in the carburetor. To eliminate the phenomena of wear or
“sticking” of the throttle valve, the same and the elements
that surround it are subjected to material hardening
treatment.

5. IDLE CIRCUIT

The working principles of all the carburetors refers to the

same physical principle. The principle is that the fuel
responds to a vacuum signal generated by the induction
action of the engine. When the gas valve is closed, or nearly
so, the depression that insists on the main spray nozzle is not
sufficient to draw fuel from the bowl. It is for this reason
that the carburetor has a second supply circuit that operates
with this kind of conditions (at IDLE) allowing the engine to
operate normally that otherwise would stop running. The
idle circuit is equipped with a supply port placed
immediately downstream of the throttle valve, at a point
such that once the valve is closed, it experiences strong
vacuum conditions and therefore is in the best condition to
supply fuel from the bowl. It is equipped with its own jet
(idling) whose choice is very important not only to work in
idle conditions but also to get the best engine response
during the first opening of the throttle valve, which
subsequently will characterize the next progression. If the
selected idle jet is too big, the engine may tend to stall and
responds slowly to the throttle. If, on the contrary, the jet is
too small, the engine responds better to the throttle but when

http://www.polini.com/en/technical-area/polini-cp-carburetors/instructions/[16.10.2017 21:20:15]
CP Instructions - Polini Com

the throttle is closed, the speed remains high for few seconds
before settling down to idle.

6. THE EMULSION AIR CIRCUIT

The fuel supplied by the idle circuit is mixed with a small

quantity of air. The quantity of air is minimal when the idle
circuit is working but it becomes more relevant for the
progression circuit when the throttle valve is lifted. The idle
set up is made by acting on the air screw.

To get an enrichment you need to screw the air screw

(close the air intake); otherwise to lean the carburation you
need to unscrew it.
The best setting is achieved with the air screw 1-1.5 turn
open from completely closed position.

7. THE CONICAL IDLE

The Polini CP carburetor is a “needle type” because by using

the conical needle you get a mechanical configuration that
ensures the correct mixture ratio for all operating conditions
of the engine. This needle is placed in a section of the
atomizer and works like this:

When the throttle valve is slightly raised the needle

reduces the space for the fuel passage and so the depression
increases but the output remains low and consequently the
mixture ratio remains fixed.
When you have a large throttle opening the needle is in the
atomizer and it increases the fuel supply space; then, while
depression is decreasing, the mixture ratio is optimal thanks
to the increase of fuel passage area. The engine is able to
operate at all throttle openings.The needle is attached to the
throttle valve with an elastic clip which binds to one of the
notches on the end of the stem. The adjustment of the needle
system can be made by acting on the needle itself. In
particular, if you need to lean the mixture you must lower
the needle (by moving the clip above) while if you have a
fuel mixture which is too rich you must lift the needle (by
moving the clip below). Even the conical degree and the
length of the conical part of the needle are important
parameters for the carburetion adjustment because they

http://www.polini.com/en/technical-area/polini-cp-carburetors/instructions/[16.10.2017 21:20:15]
CP Instructions - Polini Com

directly affect the overall engine response. When it is not

possible to correctly adjust the carburetor acting on these
parameters you will need to replace the needle with another
of different features.

8. NEEDLE-BUSHING ATOMIZER COUPLING

The adjustment of the carburetion can be done by modifying

the jets and/or changing the position and the type of the
conical needle but you can also act on the atomizer.
The atomizer end parts which is very close to the venturi has
a bush of calibrated diameter, available in different sizes. By
increasing the atomizer’s diameter, the mixture is enriched.

9. THE ATOMOZIER AND AIR BRAKE

The atomizer is a tube that connects the main jet to the

venturi and it is called “two-stroke type” when it doesn’t
have any hole. Otherwise if it has a series of holes along its
entire length it is called “four-stroke type”.(Attention: to call
the atomizers “2 or 4 stroke” is popular but technically
incorrect because you can safely use atomizers with holes
for the 2 stroke too. I.e the Minibikes have atomizers with
“4 stroke” type holes). Its working is very easy: due to the
depression in the venturi then, from the atomizer tube the
liquid fuel is drawn, metered by the main jet and by the
conical needle, while a certain airflow comes from the
channel. Air and fuel mix together and the result is a
pulverized spray sucked by the engine. To intervene in this
process, in addition to the atomizer hole, there are the air
channel diameter, the height of the part of the atomizer
protruding into the chamber and the height of the step
located in the nozzle. In addition to the atomizer’s hole
diameter, the variables are therefore the diameter of the air
channel, the height of the atomizer’s side that protrudes in
the chamber and the “step” of the delivery nozzle. The “four
stroke” type produces leaner mixture and more controlled; in
fact since it is equipped with a series of holes the emulsion
is done inside the tube, before the mixture reaches the

http://www.polini.com/en/technical-area/polini-cp-carburetors/instructions/[16.10.2017 21:20:15]
CP Instructions - Polini Com

nozzle in the venture. Obviously the arrangement of the

holes and their diameter influences the delivery. Holes
machined in the lower part of the atomizer are bathed in the
fuel of the bowl so, by increasing the numbers of holes the
flow of fuel increases and goes to emulsify itself with the
air. The holes in the upper part of the atomizer are exposed
to air and by increasing them you obtain weaker carburation
at low rpm.
The POLINI CP carburetor can be adjusted so that it works
with both the types, 2 or 4 stroke, changing the brake air jet.

Close air brake jet: 2 stroke atomizer type working as an

atomizer without holes
Open air brake jet: the atomizer holes work and make the
engine weaker.

10. THE MAIN JET

It’s one of the most important elements of the carburetor’s

adjustment, because it calibrates the fuel delivered by the
main system. To begin you have to try some jets, and it is
good to start with a bigger jet to prevent the engine damage
with a too poor fuel mixture that would lead to a seizure or
piston drilling. Proceed by trial and error: after having
driven a straight at full throttle at the highest rpm check the
spark plug color (see spark plug color table).
When you find the proper jet it is important to increase the
size of a few points as a precaution because a mixture
weakening could have passed caused by temperature
lowering or by an increase of the environmental pressure.
SPARK PLUG COLOR INDICATIONS

1. NORMAL: From dark brown to light brown with dry

electrode
2. OVERHEATED: Light grey or white Poor mixture
3. WET: Wet or sooty Reach mixture

11. COMPETITION

If the carburetor is fitted on racing engines that exceed 12000

rpm, please carefully read the following instructions. The
Polini CP carburetor is lighter than other carburetors
available on the market and under certain conditions it may

http://www.polini.com/en/technical-area/polini-cp-carburetors/instructions/[16.10.2017 21:20:15]
CP Instructions - Polini Com

cause fuel emulsion problems. Air bubbles are sucked into

the main jet casing very serious problems to the engine that
becomes poor despite having a big or correct jet; the engine
begins to detonate up to its break.
First of all check the assembly instructions in the
“Calibrations section” to ensure that the Polini CP
carburetor has been tested on your bike or scooter model by
the Polini technicians and follow the instructions.
If in the “Calibrations section” our motorcycle or scooter
model is not listed, proceed as follows:

Fit the transparent bowl (cod.343.0029) that will allow you

to detect the problem and, if necessary, soften the rubber
manifold until the problem disappears.
If the problem persists, increase the mass of the carburetor
mounting the filter coupling iron flanges (cod.343.0028)
produced by POLINI. Once the problem has been solved fit
the standard bowl again.

Home it  CERCA NEL SITO

Azienda 
Catalogo 
Gare 
Novità 
      
Rivenditori Polini 
Area tecnica 
Shop 

Copyright © POLINI MOTORI. - Tutti i diritti sono riservati. Privacy & TOS

P.IVA: IT01804650164 - Reg. Imp: 01804650164 - Bergamo - Cap. Soc. Euro 3.500.000 i.v.

http://www.polini.com/en/technical-area/polini-cp-carburetors/instructions/[16.10.2017 21:20:15]
CP Instructions - Polini Com

http://www.polini.com/en/technical-area/polini-cp-carburetors/instructions/[16.10.2017 21:20:15]