Haynes - Motorcycle Carburettor Manual.pdf - Haynes - sumo1962

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Motorcycle

Carburettor

Manual

by Pete Shoemark

ISBN 0 85696 603 7

transmitted in any form or by any means, electronic or

mechanical, including photocopying, recording or by any

information storage or retrieval system, without permission in

writing from the copyright holder.

Printed in England (603 - 1H2)

HAYNES PUBLISHING GROUP

SPARKFORD YEOVIL SOMERSET BA22 7JJ

ENGLAND

distributed in the USA by

HAYNES

PUBLICATIONS INC

861 LAWRENCE

DRIVE NEWBURY

PARK CALIFORNIA

91320 USA

There can be few motorcyclists who have not been confronted by carburation problems in one form or

another. The number of symptoms which can be attributed to some sort of carburettor malfunction is just

short of infinite, and this often leads to some tentative screwdriver-twiddling on the part of the owner, in

the vain hope that this will resolve the problem. Unfortunately, unless he’s a very lucky indeed, this will

only add another discrepancy and will often mask the true cause of the original problem. Given that many

machines possess four or more carburettors, this can lead to a very interesting tangle, and can take

hours to sort out.

This book sets out to overcome such problems, first by giving the reader a sound knowledge of the

simple principles which govern the way a carburettor functions, and then by examining in detail the more

practical aspects of tuning and correction of maladjustments. In addition to this the overhaul of the main

types of carburettors is discussed in detail as the practical use of tuning aids and equipment by which

greater accuracy can be obtained.

INCLUDED IN THIS MANUAL:

- The theory of carburation

- Carburettor development history

- Tuning procedures of the three main carburettor types

- Detailed overhaul and renovation procedures

- How to use tuning aids

- Fault finding chart

Introduction

There can be few motorcyclists who have not been con-

fronted by carburation problems in one form or another. The

number of symptoms which can be attributed to some sort of

carburettor malfunction is just short of infinite, and this often

leads to some tentative screwdriver-twiddling on the part of the

owner, in the vain hope that this will resolve the problem.

Unfortunately, unless he is very lucky indeed, this will only add

another discrepancy, and will often mask the true cause of the

original problem. Given that many machines possess four or

more carburettors, this can lead to a very interesting tangle, and

can take hours to sort out.

The root of the problem lies in the very nature of carbura

tion. It is a rather esoteric process involving the passage of

various fluids through numerous small holes. In short, a

carburettor fault can rarely be seen or measured. Most riders

could diagnose a slipping clutch with the utmost ease, and the

resulting investigation will reveal measurable wear or damage.

But when it comes to that annoying flat spot at 3000 rpm in

5th ........

This book will not show you how to measure the size of a

main jet - carburation is never that straightforward. The key to

successful carburettor timing and rebuilds is an intuitive ap-

proach, most of the fault diagnosis being done from the saddle

rather than in the workshop. To this end, it is essential that what

goes on in all those drillings, jets, air bleeds and passages is

understood, both as separate systems and as one aspect of a

rather complicated precision device. Consequently, a large

proportion of this book deals with the requirements of carbura-

tion and the various ways in which the hundreds of examples of

carburettors meet them.

The chapters relating to tuning are divided into the three

basic types, these being Slide, Constant Depression and Fixed

Jet. Further chapters relate to overhaul, specific variations from

the common designs, and to tuning aids and equipment.

It is hoped that this book will lead to a better understanding

of carburettors and of the ways in which the various types set

out to solve common problems in a variety of ways. Armed with

this, carburettor tuning should become far less mysterious and

much more accurate.

Acknowledgements

My thanks are due to the many companies who assisted

with technical advice and information during the origination of

this book. In particular, Mr B. Johnston, Technical Manager of

Amal Ltd, provided invaluable advice and assisted in checking

the text for accuracy. Robin Chan of Contact Developments

supplied technical assistance and literature on Dellorto instru-

ments. Mr G. Unsworth, Marketing Director of Gunson's Col-

ourplugs Ltd, supplied information and photographs of his

company's test equipment.

Numerous companies provided many of the line drawings

used throughout the book; I am indebted to the following:

Amal Ltd, Contact Developments, Frank T. Miyake (Presi-

dent of FTM Associates), Kawasaki (UK) Ltd, Heron-Suzuki

(GB) Ltd, Mitsui Machinery Sales (UK) Ltd, Honda (UK) Ltd,

Motobecane SA, BL Cars Ltd and NGK Spark Plugs (UK) Ltd.

Les Brazier arranged the cover photograph and those which

accompany the text. The latter are reproduced courtesy of the

Haynes Publishing Group Ltd. Jeff Clew provided invaluable

archive material and technical guidance. Tony Tranter, Principal

of Merton Technical College, supplied the sectional carburettors

featured in the cover photograph.

Mansur Darlington edited and prepared the book, and

applied the persistent pressure necessary to steer it to comple-

tion. Angie provided the necessary tea and sympathy during the

book's origination.

Many thanks to all concerned.

Contents

Introduction

Acknowledgements

Chapter 1 The Demands and the Developments

Section

Chapter 2 Carburettor types

1 Introduction

2 Operatingprinciples

3 The development of the slide carburettor

11-13

15-28

1 Preliminary checks

2 Checking the float height

3 Tuning single carburettors - four stroke engines

4 Tuning single carburettors - two-stroke engines

5 Tuning twin carburettors

6 Tuningmultiplecarburettors

29-32

32-37

38-39

39-41

41-45

Chapter 5 The Constant Depression carburettor

47-51

Chapter 6 Tuning CD Carburettors

Section

53 53

53-61

Chapter 7 The Fixed Venturi Carburettor

1 Preliminary checks

2 Checking the float height - Bendix and Keihin

3 Fuel diaphragm - Tillotson carburettors

4 Tuningprocedure

5 General running problems - fixed venturi carburettors

66-69

1 Preliminary checks

2 Removing the carburettor(s) from the machine

3 Dismantling slide type carburettors

4 Dismantling CD carburettors

5 Dismantling fixed-jet carburettors

6 Examining and renovating the carburettor

7 Rebuilding the overhauled carburettor

72-82

82-87

88-91

1 Introduction

2 Mopedcarburettors

3 Acceleratorpumps

4 Float systems and baffles

5 TheLectroncarburettor

6 Airfilters

7 Exhaustsystems

8 Air cut-off valves

9 Powerjets

92-96

97-99

99-100

101-102

103

107

1 Introduction

2 Vacuumgaugesets

3 Making up a vacuum gauge set

4 Using vacuum gauges

5 Checkingvacuumgauges

6 Vacuum synchronisation - variations

7 Obtaining accurate mixture settings

8 The Colortune 500

9 Using the Colortune 500

10 Using the Colortune 500 as a diagnostic aid

11 Carburettor synchronisation without vacuum gauges

104

104-106

106

106-107

107

110

111

111-112

Chapter 12 Future Developments - the end of an era?

114

Chapter 13 Fault Diagnosis

115-116

1 The role of the carburettor

2 Fuels

3 Atheoreticaldesign

Chapter 3 The Slide Carburettor

Section

Chapter 4 Tuning Slide Carburettors

Section

1 Preliminary checks

2 Checking the float level

3 Tuningprocedure

Chapter 8 Tuning Fixed-jet Carburettors

Section

Chapter 9 Carburettor Overhaul

Section

Chapter 10 Design Variation and Ancillary Components

Section

Chapter 11 Tuning Methods and Aids

Section

Chapter 1

The Demands and the Developments

Contents

The role of the carburettor................................................. 1

Fuels .............................................................................. 2

A theoretical design ........................................................... 3

1 The role of the carburettor

ment must by now have reached a peak. This is far from the

truth, because the energy crisis which began to become

apparent during the 1970s has made it necessary to produce a

most frugal instrument, but one which is still capable of allow-

ing a high engine power output. In this way, the two most con-

tradictory requirements have been heavily underlined. The car-

burettor may be almost perfect, but the search for perfection

continues.

As motorcycle design moves into the 1980s, it has become

increasingly difficult for the manufacturer to produce the right

product. In the earliest days of motorcycling, the only route to

obtaining power was by means of large capacity engines. By

1960, engine and carburettor design had seen engine capacity

fall by almost half, and 650 cc machines, the 'big' capacity

models of that era, were able to combine a high maximum

speed with reasonable fuel consumption. Since the late 1960s,

engine capacity has climbed once more in the search for more

and more power. This new generation of large capacity

machines, mostly four- or six-cylinder four-strokes, are not the

simple low-efficiency devices of the 1920s. Engine develop-

ment has seen single or double overhead camshafts become the

norm on roadsters, and such high performance engines have a

thirst commensurate with their specification.

Increasing concern about the effects of atmospheric pollu-

tion and the earth's rapidly dwindling oil stocks has placed the

already hard-pressed carburettor in a position where it is forced

to play an even more demanding role. It is the conflicting

demands of a public who require a motorcycle to perform

better, using less fuel and producing the minimum of pollution,

which have pushed the simple slide-type carburettor to its

limits, and have brought about the introduction of a promising

newcomer to motorcycles, the CD (constant depression) or CV

(constant vacuum) instrument.

Future developments in carburation are difficult to predict.

The conflicting demands mentioned previously are ultimately

impossible to resolve, and development of existing types will

almost certainly be as a result of a change of emphasis of these

requirements. A more economical machine is quite feasible, but

power outputs must fall accordingly.

Alternatives are not numerous. The fixed-jet carburettor,

still favoured by the car manufacturers, has not generally found

a receptive home on the motorcycle. It has a number of

drawbacks, amongst them cost and fuel economy considera-

tions, and it seems unlikely that this type of instrument has

much future where motorcycles are concerned.

The only obvious innovation may be fuel injection. This

system does not constitute carburation in the accepted sense,

and has yet to prove itself as a commercial success. On both

Every motorcycle engine, from the simple single-cylinder

two-stroke, to the most sophisticated multi-cylinder four-stroke,

is dependent on two very precise pieces of ancillary equipment.

The first of these, the ignition system, is of obvious importance

because it supplies the exactly-timed spark which ensures that

combustion occurs at precisely the right moment. The second

piece of equipment can fairly be considered to be of even

greater importance, for without it, the engine cannot be run or

controlled. It is, of course the carburettor.

During every engine cycle, be it two- or four-stroke, the car-

burettor must feed the engine with a precise amount of fuel,

mixed with an equally precise amount of air. Moreover, as

loading on the engine varies, this fuel mixture must be varied to

compensate.

When the engine is cold the ratio of fuel to air must be

altered radically; when idling, the carburettor must function

automatically; and when it is wished to increase the speed of

the engine, some means of controlling the carburettor's opera-

tion to fine limits must be contrived.

It will already be apparent that the carburettor must be

capable of performing a wide range of functions with great

accuracy and consistency, allowing induction to take place as

often as ten thousand times every minute or even more. Equally

important, it must be robust, to endure extremes of temperature

and vibration and wide variations of climate.

It is not surprising, therefore, that the instrument that we

tend to accept today without a second thought has taken

almost a century to evolve. The earliest carburettors were crude

and inefficient and were a major limitation in producing useful

amounts of power from early engines. This problem was

painfully evident to the early engine designers, and from the

plethora of carburettor designs there soon emerged the recog-

nisable ancestors of the three basic designs in common use

today. The intervening years have provided ample opportunity

for refinement and improvement.

It is not unreasonable to say that the carburettor is amongst

the most highly developed aspects of the internal combustion

engine. Having conquered the fundamental problems of supply-

ing a combustible mixture to the engine, the carburettor

manufacturers were obliged to engage themselves in a

relentless search for sophistication. It was obvious that the

motorcycle manufacturer would choose only those instruments

which made the most of his machine, and despite widely con-

flicting demands, only a few specialised carburettor companies

have survived to this day.

There is a tendency to assume that carburettor develop-

Chapter 1 The Demands and the Developments

counts, it is not the concern of this book, but it may well be the

shape of things to come. The basic principles of fuel injection

are discussed briefly elsewhere in this book.

To understand the various types of carburettor used on

modern motorcycles, it is important to understand first what is

required of this instrument, and how this can be accommodated

by the basic, theoretical, carburettor. Do not be tempted to skip

this part of the book and move on to the practicalities of the

various types — the theory is essential to tuning, as it is

important to understand exactly what effect any adjustment

may have, and why this should be so. Armed with this

understanding, attempts at tuning will become more intuitive

and accurate. Trial-and-error can only be more costly and time

consuming in the end.

than has been explained, but as this book relates to carburettors

rather than fuel, and to keep the subject as digestible as

possible, further exploration of this rather involved subject will

be avoided.

Having found a good fuel, it is necessary to mix it with a

suitable amount of air to achieve efficient combustion. It can be

demonstrated that any given quantity of fuel will require a fixed

proportion of air to effect full combustion. This ratio is of vital

importance, as a significant variation either way will result in

wasted potential on each power stroke. If excess air is admitted, a

proportionately smaller amount of fuel can be admitted, and

consequently, power is less than is possible. Conversely, if too

much fuel is fed to the engine, there will not be sufficient air to

allow it to be burnt, and again, power is lost.

These two extremes are what are known respectively as

weakness or richness. There is, naturally enough, a position

between these extremes where the mixture is correctly

balanced. In practice, this is a ratio of fifteen units of air to every

one of fuel, or 15:1, as an optimum balance between perfor-

mance and economy. The absolute practical limits are between

12:1 and 18:1, and the mixture can be varied between these

two ratios to obtain either better performance or economy.

The implications of an extremely weak or extremely rich

mixture are more significant than might at first be suspected.

Most motorcyclists will be vaguely aware that these conditions

do little to enhance an engine's longevity, but may not know

exactly what goes on inside the cylinder in these circumstances.

When an engine is run with a weak mixture, the sparsity of

fuel in the combustion space means that the time taken for

combustion is longer. The flame front spreads relatively slowly,

and may even still be burning as the piston reaches the bottom of

its stroke. This causes the engine to run abnormally hot (due to

the increased time available for heat transfer to the cylinder

components) and, in extreme cases, the valves on four-stroke

engines, and the pistons on two- or four-stroke engines, may

become burned. Two-stroke engines are particularly susceptible to

weak mixtures, and it is this and incorrect ignition timing which

is the main cause of holed pistons.

With an over rich mixture, carbon build-up within the

engine becomes greatly accelerated, and frequent fouling of the

sparking plug is often evident. In extreme cases 'fuel-wash', a

condition where the excess petrol dilutes the oil film on the

cylinder walls, can cause premature wear or even seizure.

Other effects of incorrect mixture ratios are the emission of

increased quantities of toxic gases from the engine's exhaust.

This factor is of growing significance now that laws governing

exhaust emission have been passed in many countries. It is

obvious that a rich mixture will result in poor fuel economy, but

perhaps surprising that a weak mixture can have the same

result due to the inefficiency of the engine in these conditions.

It will be appreciated from the foregoing that it is essential

that the fuel-air ratio is maintained to fine tolerances if the

engine is to operate efficiently. When an engine type is first built

carburation is a major factor in subsequent development and

testing. Most manufacturers now use sophisticated equipment

which electronically analyses the exhaust gases, thus assessing

the mixture strength very accurately. This is normally done in

conjunction with a rolling road, and thus the carburettor can be

chosen, adjusted and tuned very accurately indeed in the

development workshop.

Scaled-down versions of these complex diagnostic

machines are now in common use in large motorcycle dealers,

enabling the mechanic to tune the engine to perfection, when

used astutely. This equipment is complex, bulky and frighten-

ingly expensive, but by no means essential to the home

mechanic. In many cases the same degree of accuracy can be

obtained by less glamorous means, even if it does take a bit

longer. It is amusing to note that an article in a British

motorcycle magazine found that the ignition timing on their test

machine could be set using a slip of cigarette paper with the

same degree of accuracy as a diagnostic machine costing

several thousands of pounds. The same can be applied to tuning

and maintaining carburettors.

2 Fuels

This book will concentrate almost solely on petrol, or gas-

oline, as a fuel. This is simply because it is almost exclusively

used as the power source for road-going machines. When asked

why this should be the case, it is tempting to reply that petrol is

easily obtainable. This really only states that petrol is generally

accepted as a good all-round fuel for road use. If this were not

the case, we might have pumps on garage forecourts to serve us

with paraffin (kerosene) LPG (liquid petroleum gas) or any one

of a number of petroleum fractions.

The liquid that is pumped into our fuel tanks - 'petrol' or

'gasoline', depending on one's location in the world — tends to

get dismissed with no thought given to why it is used or from

where it is obtained. The beginning of the story is that increas-

ingly valuable commodity, crude oil. This is extracted in

numerous places around the world and shipped or piped to oil

refineries for processing. It is of such importance that its price

and availability is a major factor in the world's economics.

Crude oil is a thick black substance, and it can be rather

difficult to relate the black tarry mess which washes up on our

beaches from time-to-time, with the aromatic liquid that got us

to the beach in the first place.

Crude oil is broken down by distillation in oil refineries, to

produce the numerous oil-based products that we use every

day. In a simplified form, what happens is this: the crude oil is

heated at the base of a large tower, causing it to evaporate and

move upwards. The tower is divided into galleries which are

maintained at fixed temperatures, becoming progressively

cooler towards the top. The lightest fractions of the crude oil

remain gaseous until they reach the top of the tower, the

remaining, less volatile, compounds condensing at progressively

lower levels, and at higher temperatures. The condensates are

then drawn off from the separate levels.

What we require at the petrol pump must fulfil a number of

conflicting criteria. The fuel must have a moderately high

Calorific Value, or CV. This is the amount of heat energy per

unit of fuel, and within limits, the higher this factor is, the more

is the work that can be obtained from a given amount of fuel.

Unfortunately, the higher the CV, the higher the density of the

fuel, and consequently, the lower its Volatility. A highly volatile

liquid is one which will evaporate readily at comparatively low

temperatures, and this is in practice at the other end of the

range to the fuels of a high CV. So it can be seen that the choice of

fuels has been narrowed considerably, and must lie as a com-

promise between a fuel of high CV and one of high volatility. A

third factor which influences the choice of fuel is its resistance

to Detonation, or self-ignition under pressure. Because fuels

are inevitably compressed in the cylinder, low anti-knock

characteristics must be avoided at all costs. Detonation will

happen when a high-compression engine is run on low-grade

fuel, and will ultimately destroy the engine.

Given the foregoing requirements, we find ourselves with

common pump petrol, or gasoline, as the obvious choice for our

fuel. There are a number of good alternatives, but these can

largely be ruled out on grounds of cost or convenience. The

question of fuel choice is in fact a good deal more complicated

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