Table of Contents Table of Contents
Previous Page  545 / 612 Next Page
Show Menu
Previous Page 545 / 612 Next Page
Page Background

This manual us untended as a guide for users of Mikuni

carburetors who want to learn the basic methods of tuning and

adjusting to obtain top performance and fuel economy. The arrows

that appear in the drawings in this text show direction in which air,

fuel, and air-fuel mixture lows.

Information here with in obtained from Mikuni

engineering data and manuals.


The function of a carburetor is to deliver a combustible air-fuel mixture

to the engine. However, in order to be effective, it must irst break the

fuel unto tiny particles (in the form of vapor) and then mix the fuel with

air in a proper ratio so it can burn without leaving excess fuel or a



The mixture of the air-fuel ratio is generally expressed by its relative

weight proportion. For example, the amount of air required for

complete combustion of 1 gram of fuel under normal condition is:

Varying mixture ratios are required for the engine depending on

operating conditions. Although the required mixture ratio varies more

or less with the engine, its colling eficiency, etc., the mixture

ratio shown in ig. 1 is required for ordinary engines. In the high speed

range the ratio of about 12 to 13 grams of air for 1 gram of fuel

produces the maximum output. However, in the case of an engine with

low cooling eficiency, somewhat richer mixture (10 to 12 grams of

air against 1 gram of fuel) may be required to prevent seizure of the




Motorcycle engines are operated under a wide range of conditions,

from idling with the throttle valve (Fig.2(1)) remaining almost closed,

to the ill load (the maximum output) with the throttle valve fully

opened. In order to meet the requirements for the proper mixture ratio

under these varying conditions, a low-speed fuel system (the pilot

system) and a main fuel system (the main system) are provided in

Mikuni Slide -type carburetors, except Mikuni TMS.

A. - The Pilot System

Low -speed fuel system (Fig. 2 and Fig.3)

Since the engine is operated with the throttle valve almost closed at

idling or in the low speed range, the velocity of air lowing through

the needle jet (2) is sloe. Consequently, a vacuum strong enough to

draw fuel from the needle jet in the main fuel system is not created. The

fuel supply during the slow speed operation is controlled by means of

the pilot outlet (3) and the bypass (4) that are situated nearest to the

engine. At idle, when the throttle valve is slightly opened, fuel

metered by the pilot jet (5) is mixed with air adjusted in a proper

amount by the air screw (6) and is broken into ine vapor particles.

The mixture is again mixed with air coming from the bypass and is

drawn into the pilot outlet to mix air lowing through the main bore (7).

The fuel mixed with air at this stage then goes to into the engine. When

the throttle valve is opened slightly during low speed operation, the pilot

outlet alone cannot supply the

required fuel and the shortage

has to be made up with fuel

injected from the bypass. The

adjustment of the mixture

ratio during this stage is

made by the pilot jet and the air

screw, as in the case of a

two-hole type fuel system

(Fig. 3). While at low speed

operation, if full throttle is

initiated a similar shortage of

fuel again has to be injected

from the bypass until enough

(vacuum) can be created to

draw fuel from the main fuel

system. There is also a one-

hole type low speed fuel sys-

tem mainly used for carburetors

having a small main bore. The

process of producing the air

fuel mixture ratio are the same

as in a two-hole type low speed


General Mikuni Slide Carb Circuitry

Air-fuel mixture

Maximum power mixture ratio

Theoretical mixture ratio

Throttle valve opening (%)

20 40 60 80 100

15 gram of

1 gram of



= Mixture







Figure 1

Figure 2

Figure 3