TWO STROKE TUNING

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

amazed at the gains to be made, particularly in reliability, and to a lesser extent in

performance, by correcting manufacturing deficiencies. I am convinced manufacturers

bolt their road racers together merely to make shipping all the pieces easier, such are

their tolerances.

I have seen engines that have never been started with piston clearances larger than

the manufacturer's serviceable limit. Conrods that vary 0.4mm in centre to centre

length and 20 grams in weight, on the same crank. Crankwheels which are 0.1mm

outside true centre. Cylinder heads with a squish band clearance of 1.7mm, instead of

0.7-1.0mm. Cylinders with port edges so sharp that the side of the piston and rings

would have been shaved away in a few minutes' running. New pistons with cracks. New

cylinder heads that are porous.

Included in blueprinting is cleaning the rough cast out of the ports, and matching

all gaskets so they don't overlap the ports. The transfer ports must be matched to the

crankcase. The carburettor should coincide with the mounting flange and inlet port.

Anything the manufacturer has not done (presumably to cut costs), you should do.

Blueprinting is slow, tedious work, and it can be expensive when crankshafts have

to be separated and then machined and trued, or when cylinder heads have to be

machined to close up the squish band without raising the compression ratio. It is not

very exciting work because when you have finished the engine is stock standard, and

telling your mates all the work you have done won't impress them. But don't let this

put you off, the basis for any serious tuning must begin with bringing the engine up to

the manufacturer's specifications.

Most people won't believe how close to standard are the motors used by the

factory racing teams. Other riders are convinced that, because the factory boys are

quicker, they must have more power and lots of trick parts. In truth, the differences are

in frame geometry and the ability of the factory rider to ride faster and make the right

choice of tyres, suspension settings, gearing, jetting etc. Plus, of course, they use

blueprinted engines.

So that there is no misunderstanding of the two-stroke operating cycle I will

describe what goes on in each cylinder, every revolution of the crankshaft.

The first example is the piston-ported Bultaco Matador Mk4 which, like most

modern two-strokes, operates on the loop scavenge principle. As the piston goes up,

the inlet port is opened by the piston skirt at 75° before TDC (Top Dead Centre) and

the atmospheric pressure (14.7 psi) forces air/fuel mixture in to fill the crankcase

(FIGURE 1.1). The piston continues to rise to TDC, compressing the fuel/air charge

admitted on the previous cycle. At 3.2mm before TDC the spark plug fires, sending the

piston down on the power cycle. As the piston continues its descent the inlet port is

closed and the fuel/air mixture is partially compressed in the crankcase. 85° before

BDC (Bottom Dead Centre) the exhaust port is opened by the piston crown and the

exhaust gases flow out. After another 22° (63° before BDC) the blow-down period

finishes and the piston crown exposes the transfer ports to admit the fresh fuel/air

charge. This is forced up the transfer passages due to the descending piston reducing

the crankcase volume by the equivalent of the cylinder displacement, in this instance

244cc. As the piston begins rising, the mixture continues to flow into the cylinder and

the exhaust gas continues flowing out. The piston continues rising, closing off first the

transfer and then exhaust ports. Next the inlet port opens, to start the cycle over again.

10 Rotary valve engines operate on the same loop scavenge principle, but in this case

Introduction

A -mixture in cylinder is compressed & inlet cycle begins.

B-mixture in crankcase is compressed.

C- exhaust cycle begins & primary compression continues.

D- transfer cycle begins & exhaust cycle continues.

Fig. 1 .1 Basic two stroke operation. 

a disc partially cut away and attached to the end of the crankshaft opens and closes an

inlet port in the side of the crankcase. The Morbidelli 125 twin road racer is a rotary

valve engine. The inlet port opens 30° after BDC and closes 79° after TDC. The piston

crown opens and closes the exhaust and transfer ports.

The following pages will provide you with the knowledge necessary to develop a

successful two-stroke competition engine, but do keep in mind the principles outlined

in this chapter so that you avoid the most basic pitfalls associated with two-stroke

tuning.