two_stroke_performance_tuning.pdf

Contents

Page

Preface

Chapter 1 —

Introduction

Chapter 2— The cylinder head

Chapter 3 — Porting and cylinder scavenging

Chapter 4 — The exhaust

Chapter 5 — Carburation

Chapter 6 — Ignition

125

Chapter 7— The bottom end

143

Chapter 8— Lubrication and cooling

166

Chapter 9— Power measurement and gearing

174

Appendix I — Introduction

187

A Motocross modifications

187

B Enduro modifications

198

C Road race modifications

202

Appendix II— Table of useful equivalents

219

Appendix III—Speciality suppliers

•

220

Index

222

Preface

FROM A very humble beginning, the two-stroke internal combustion engine has

now been developed to a degree that was not thought possible just a few years ago. I

am sure even the engineers who have stood by the two-stroke principle for so long find

it staggering that this mechanically simple device can produce as much power as it does

today, with relative reliability.

Originally, I looked upon two-stroke engines with contempt. They made a horrible

ring-ding noise, nothing like the beautiful note of four-stroke racing engines. They

emitted a blue haze from their tailpipes too, which appeared unsightly, long before any

of us heard of the word pollution. On hot days these engines seized with monotonous

regularity. Difficult starting, flooding and plug fouling seemed the order of the day.

Consequently I wrote off two-strokes, convinced I would never lower myself to

develop one of these unreliable little beasts in my workshop. But that all changed when

two of my friends bought themselves 250cc Bultaco Pursang motocross bikes and

insisted I prepared them. I took up the challenge and was rewarded with the knowledge

that a ring-ding I had developed came home 3rd in the National Motocross

Championship with a B grade rider on board.

From then on the challenge has not abated as I have strived to unravel the mystery

of what makes a two-stroke tick. Instead of looking on the two-stroke with contempt, I

now view this little marvel with fascination. Four-stroke engine development has just

about reached its peak, but there is much yet to be learned about the two-stroke power

unit.

It is my hope that this book will assist the enthusiast involved in motocross,

enduro, desert, road or go-kart racing to develop and tune his two-stroke engine for

horsepower and reliability.

Maitland

New South Wales

Chapter 1

Introduction

MECHANICALLY, the two-stroke engine is very simple, and unfortunately on

too many occasions this apparent simplicity has fooled would-be tuners into believing

that this type of power unit is easy to modify. Just a few hours work with a file in the

exhaust and inlet ports can change the entire character of the engine for the better, but

if you go just 0.5mm too far, you could end up with a device slower than its stock

counterpart.

Therefore modifications must be planned carefully, keeping in mind that seldom,

if ever, is the biggest (or most expensive) the best. As you plan your modifications

always tend to be conservative. If necessary, you can go bigger later.

Possibly the worst viewpoint you can start out with is that the manufacturer didn't

know what he was doing. I started out thinking that way too; but then I began to realise

why the engineers did it that way. Pretty soon I was learning more about what makes a

two-stroke fire — and making fewer mistakes.

You must keep in mind that all production engines are a compromise, even highly

developed racing engines like the Yamaha TZ250. You can make the TZ churn out

more power, but will you be able to ride it with the power band narrowed right down,

and do you have the experience to handle a sudden rush of power at the top end on an

oily or wet track? Also, think about the added wear caused by more rpm and

horsepower; do you have the finances to replace the crankshaft, pistons and cylinder

more frequently now that you are running at 12,500rpm instead of 11,500rpm? When

you begin to think about things like this, you start to understand a few of the reasons

why manufacturers make compromise engines and machines. Remember the TZ250

started out as a road racer, so you can imagine some of the problems you could come

up against if you were to modify a single cylinder 125 motocross engine for use in a

road racer.

Obviously the first work you should do is bring the engine up to the

manufacturer's specifications. This is termed blueprinting, and involves accurately

measuring everything and then correcting any errors made in production. You will be 9

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