3368441308-PZ Carb Tuning Manual.pdf

PZ CARBURETOR

PERFORMANCE TUNING

AND MAINTENANCE

PZ CARBURETOR

PERFORMANCE TUNING

AND MAINTENANCE

All information provided is intended for general guidelines and

practices for Basic Operation, Tuning, and Maintenance of Walbro

PZ Slide Valve Carburetors.

Contents:

SAFETY

TUNING

Idle and Low Speed

Midrange/Part Throttle

High Speed/Full Throttle

TUNING TIPS

SET-UP

MAINTENANCE

SPECIFICATIONS

NOTES

EXPLODED VIEWS

SAFETY

As with any carburetor or fuel system, care must be taken to avoid ﬁ re from leaking or spilled fuel. Be

sure all gaskets and seals are in good condition and properly assembled. Dispose of any drained or spilled fuel.

Check condition of fuel lines and connections before every use. Fuel must be drained from the system if vehicle or

engine is not stored or transported in a level position.

It is necessary to check the carburetor slide for free movement through the full range of travel any

time the carburetor has been removed, installed, or adjusted and before starting the engine . Remove

air ﬁ lter and visually conﬁ rm the slide valve is fully closed as the throttle is released. Work the throttle control from

closed to fully open and watch for any sticking or hang-up. Engines could start with an open throttle resulting in a

runaway vehicle. Always install the correct return spring for the carburetor and use an additional return spring on

the throttle pedal as needed.

Motorsports are dangerous and can lead to injury or death.

PERFORMANCE TUNING 2

TUNING

PZ22

Rich and Lean: An internal combustion

engine needs a speciﬁ c ratio or

mixture of air and fuel for best

performance. This mixture is delivered

by the carburetor. In the following

discussion numerous references will

be made to the air and fuel mixture

(air/fuel ratio) being rich or lean. Rich

refers to a mixture with an excess of

fuel. Lean refers to a mixture with an

excess of air.

Float height

Parts involved:

• Float

• Inlet Needle

The ﬂ oat height controls the fuel level

in the ﬂ oat bowl. This adjustment

determines the point at which the inlet

needle opens as the fuel level drops

in the bowl. Float height is usually

measured with the ﬂ oat bowl removed

and is speciﬁ ed from the carburetor

body to an edge or surface on the

ﬂ oat. The ﬂ oat should be resting on

the inlet needle but not depressing the

spring under the needle stem when

this measurement is taken. To adjust,

carefully bend the tab where it makes

contact with the inlet needle.

PZ26

The ﬂ oat height should be adjusted

before any other adjustments are

made. A higher fuel level will cause

a richer mixture and possibly excess

overﬂ ow from the vents. A lower fuel

level will a cause a leaner mixture

and could lead to an engine miss

or cut-out while cornering. Start at

the speciﬁ ed height and change as

needed for best performance on

different type of tracks.

The ﬂ oat drop should also be

checked. The proper adjustment will

prevent the needle from pulling too

far out of the inlet seat and becoming

stuck in the open position.

PERFORMANCE TUNING 3

Idle and Low Speed

stops. Back out the speciﬁ ed number

of turns. The optimum setting of the

mixture screw will usually result in the

highest rpm with the throttle closed.

As the amount of fuel is varied the

engine rpm will also vary. With throttle

closed and engine warm and running

at or slightly above desired idle speed,

slowly turn the Idle Mixture Screw in

or out to obtain the highest rpm. Then

set the idle speed screw to obtain the

desired rpm.

PZ22

Parts involved:

• Idle Speed Screw

• Idle Mixture Screw

• Pilot Jet (Idle Jet, Slow Jet)

Idle

Speed

Screw

Carburetor tuning at idle and low

speed is accomplished by adjusting

the Idle Mixture Screw, the Pilot (or

Idle) Jet, and Idle Speed Screw. These

adjustments will control idle speed,

idle quality, and initial responsiveness/

acceleration of the engine.

If a smooth idle cannot be set with the

idle mixture screw between ¼ to 2-¼

turns out, or if the engine does not

respond to adjustment, a different size

Pilot Jet may be needed. The pilot jet,

also called idle or slow jet, controls

the amount of fuel at idle and throttle

positions up to ¼ open.. If the mixture

needle cannot be leaned out enough,

a smaller Pilot Jet should be installed.

A larger jet should be installed if

the mixture is too lean. The proper

Pilot Jet size will allow a satisfactory

setting with the mixture screw.

The screw in the center of the side

of the carburetor body is the Idle

Speed adjustment. This screw holds

the throttle slide valve open slightly

to obtain the desired engine rpm

at closed throttle. Turning the idle

speed screw clockwise will increase

rpm. Turning counter-clockwise will

decrease rpm.

Idle

Mixture

Screw

The Pilot Jet is screwed into the

carburetor body and is located inside

the ﬂ oat bowl. (See photo, next page.)

The Pilot Jet meters the amount of

fuel entering the low speed circuit. A

larger or smaller jet will change fuel

ﬂ ow accordingly. The size of the jet in

millimeters is stamped on it.

PZ26

��

Idle

Speed

Screw

The Idle Mixture Screw can be either

an air or fuel adjustment so it should

be determined what type of change

you are making. A mixture screw

on the air ﬁ lter side of the PZ22

carburetor is an air-only adjustment

so opening the screw (turning

counterclockwise) will increase the

air bleed and lean the mixture. The

mixture screw on the engine side

of the PZ26 carburetor is a fuel

adjustment. Opening the screw will

allow more fuel into the engine for a

richer mixture.

Idle

Mixture

Screw

For an initial setting, gently turn the

mixture screw in until it lightly seats or

PERFORMANCE TUNING 4

stops. Back out the speciﬁ ed number

Midrange/Part Throttle

To richen the part throttle operation,

move the clip to the next lower notch.

This will hold the needle farther out

of the main nozzle and allow more

fuel into the air stream. To lean part

throttle operation move the clip to the

next higher position. The notch at the

top of the needle would be considered

the ﬁ rst position (leanest). Needle

taper reference letters are stamped on

the needle for identiﬁ cation.

Part involved:

• Jet Needle

• Main Nozzle (Needle Jet)

• Throttle Slide Valve

The Jet Needle primarily controls fuel

ﬂ ow between 10% and 75% throttle

opening. The Jet Needle is the long

narrow needle in the center of the

throttle slide valve. The needle can

be seen when the throttle is opened.

It is held in place in the center of the

throttle slide by a V-shaped clip.

Location of C-clip on jet needle determines

mid-range fuel mixture.

The Jet Needle has ﬁ ve notches

and a C-clip which determines the

position of the needle in the slide

based on which notch the clip is

placed in. By varying the height of the

needle in relation to throttle opening,

the amount of taper of the needle in

the main nozzle will change the fuel

mixture.

RAISING THE NEEDLE IN THE SLIDE = RICHER MIXTURE

LOWERING THE NEEDLE IN THE SLIDE = LEANER MIXTURE

Be sure the V-shaped jet needle retaining clip is

in place and will hold the jet needle securely in

the slide.

High Speed/Full Throttle

Parts Involved:

• Main Jet

The Main Jet controls the fuel ﬂ ow

at throttle positions of one-half to full

throttle. The Main Jet is located inside

the ﬂ oat bowl and is screwed into the

bottom of the Emulsion Tube in the

center of the carburetor body. The

jet size or diameter in millimeters is

stamped on the jet. A larger diameter

jet allows more fuel ﬂ ow and a richer

mixture.

PZ22

PZ26

Main Jet

Pilot

Jet

Main Jet

Pilot

Jet

Altitude and weather conditions can

affect the engine operation enough to

require changing the size of the Main

Jet. High air temperature, humidity,

or altitude could require a smaller

Main Jet. Low temperature, humidity,

or altitude would require a larger

diameter Main Jet

RUNNING THE ENGINE WITH AN IMPROPER MAIN JET COULD RESULT IN A LOSS

OF POWER, HIGH ENGINE TEMPERATURES, OR ENGINE DAMAGE.

PERFORMANCE TUNING 5

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