Manual Yamaha YZ125 (page 156 of 168) (English)

514481

156

Zoom out

Zoom in

1/168

7-1

ENGINE

TUNING

ENGINE

CARBURETOR SETTING

• The role of fuel is to cool the engine,

and in the case of a 2-stroke en-

gine, to lubricate the engine in addi-

tion to power generation.

Accordingly, if a mixture of air and

fuel is too lean, abnormal combus-

tion will occur, and engine seizure

may result. If the mixture is too rich,

spark plugs will get wet with oil, thus

making it impossible to bring the en-

gine into full play or if the worst

comes to the worst, the engine may

stall.

• The richness of the air-fuel mixture

required for the engine will vary with

atmospheric conditions of the day

and therefore, the settings of the

carburetor must be properly suited

to the atmospheric conditions (air

pressure, humidity and tempera-

ture).

• Finally, the rider himself must make

a test-run and check his machine

for conditions (pick-up of engine

speed, road surface conditions)

and for the discoloration of the

spark plug(s). After taking these

into consideration, he must select

the best possible carburetor set-

tings.

It is advisable to make a note of set-

tings, atmospheric conditions, road

surface condition, lap-time, etc. so

that the memorandum can be used

as a reference useful for future.

ATMOSPHERIC CONDITIONS AND

CARBURETOR SETTINGS

The reason for the above tendency is

that the richness or leanness of a fuel

mixture depends on the density of the

air (i.e. the concentration of oxygen in

it).

• Higher temperature expands the air

with its resultant reduced density.

• Higher humidity reduces the

amount of oxygen in the air by so

much of the water vapor in the

same air.

• Lower atmospheric pressure (at a

high altitude) reduces the density of

the air.

TEST RUN

After warming up the engine

equipped with the standard type car-

buretor(s) and spark plug(s), run two

or three laps of the circuit and check

the smooth operation of the engine

and discoloration of spark plug(s).

A. Normal

B. Over burned (too lean)

C. Oil fouled (too rich)

EFFECT OF SETTING PARTS IN

RELATION TO THROTTLE VALVE

OPENING

A. Closed

B. Full-open

1. Pilot air screw

2. Pilot jet

3. Jet needle

4. Diameter of straight portion

5. Clip position

6. Throttle valve

7. Main jet

ADJUSTING THE MAIN JET

The richness of air-fuel mixture with

1/2–4/4 throttle can be set by chang-

ing the main jet "1".

1. Spark plug is too hot.

• Select a main jet having higher

calibrating No. than standard. (To

be enriched)

2. Spark plug is wet.

• Select a main jet having lower cal-

ibrating No. than standard. (To be

leaned out)

ADJUSTING THE PILOT AIR

SCREW

The richness of the air-fuel mixture

with full closed to 1/4 throttle can be

set by turning the pilot air screw "1".

Turning in the pilot air screw will en-

rich the mixture at low speeds, and

turning out it will lean out the mixture.

Air

temp.

Hu-

midity

Air

pres-

sure

(alti-

tude)

Mix-

ture

Set-

ting

High High

Low

(high)

Rich-

Lean-

Low Low

High

(low)

Lean-

Rich-

Discoloration

Condition of spark

plug

Normal

Insulator is dry

and burnt brown.

Over burned

(too lean)

Insulator is whit-

ish.

Oil fouled (too

rich)

Insulator is sooty

and wet.

Standard main jet #430

Standard pilot

air screw posi-

tion

2-1/4 turns out

Report abuse

Libble takes abuse of its services very seriously. We're committed to dealing with such abuse according to the laws in your country of residence. When you submit a report, we'll investigate it and take the appropriate action. We'll get back to you only if we require additional details or have more information to share.

Product:

Forumrules

To achieve meaningful questions, we apply the following rules:

First, read the manual;
Check if your question has been asked previously;
Try to ask your question as clearly as possible;
Did you already try to solve the problem? Please mention this;
Is your problem solved by a visitor then let him/her know in this forum;
To give a response to a question or answer, do not use this form but click on the button 'reply to this question';
Your question will be posted here and emailed to our subscribers. Therefore, avoid filling in personal details.

new questions and answers
new manuals

You will receive an email to register for one or both of the options.

Get your user manual by e-mail

Enter your email address to receive the manual of Yamaha YZ125 in the language / languages: English as an attachment in your email.

The manual is 8,86 mb in size.

You will receive the manual in your email within minutes. If you have not received an email, then probably have entered the wrong email address or your mailbox is too full. In addition, it may be that your ISP may have a maximum size for emails to receive.

Others manual(s) of Yamaha YZ125

Yamaha YZ125 User Manual - German - 172 pages

The manual is sent by email. Check your email

If you have not received an email with the manual within fifteen minutes, it may be that you have a entered a wrong email address or that your ISP has set a maximum size to receive email that is smaller than the size of the manual.

The email address you have provided is not correct.

Please check the email address and correct it.

Your question is posted on this page

Would you like to receive an email when new answers and questions are posted? Please enter your email address.

Info

Yamaha YZ125 Manual

Product: