TWO-STROKE    engines    with    flat-
topped pistons, of which the latest 
Villiers Junior de luxe engine is an 
example, differ from the more familiar
deflector piston type chiefly in the design
of the ports and in the characteristics of 
their performance. The operating principles 
are, broadly speaking, the same.
 For the benefit of those whose experience 
has hitherto been confined to purely 
pedal-propelled bicycles an explanation of 
the functioning of the two-stroke engine is 
included.

HOW THE TWO-STROKE WORKS

A flat-topped piston in a cylinder is connected 
to a crank. As the crank is turned 
through one revolution the piston travels 
once down and once up. Every down 
stroke is a firing stroke, i,e., there is an 
explosion of gases every two strokes of 
the piston as distinct from once in every 
four on a four-stroke. In other words, 
there is one explosion in the cylinder for 
every revolution of the crank.
 If a study is made of the diagrams which 
illustrate the cycle of operations it will be

         TABULATED   DATA
   VILLIERS JUNIOR-DE-LUXE ENGINE
Bore and stroke.............. 50 mm. x 50 mm,
Capacity......................98 c,c.
Sparking Plug : type .......Lodge CB 3, 18 mm.
                gap............0.020"
Contact-breaker gap........... 0.015"
Piston : skirt clearance.......0.004"--O.O05"
              Oversizes........0.015"--0.030"
Chain (endless):  pitch .......3/8"
                  width .......0.155"
            No. of links.......54

Recommended 
  lubricants: Engine ,        Patent Castrol XL
              Chaincase        Castrol D 

appreciated that compression of the gas 
takes place in the crankcase below the 
piston as well as in the combustion head 
above it. It is essential, therefore, that 
the crankcasc is gas tight.
There are seven ports in the cylinder, 
made up as follows, one inlet, two exhaust,
and four transfer. The exhaust ports are 
diametrically opposite ; the inlet is im-
mediately underneath one of the exhaust 
ports and the transfer ports are in pairs 
arranged between the exhaust ports. The

gas from the carburettor is drawn into the 
crankcase through the inlet port, is trans-
ferred to the combustion space  by   the 
transfer ports, and is expelled  as  burnt
gases via the exhaust ports.
 The cycle of operations is as follows :—
 (1) The  piston  rises,  uncovering  the 
inlet port, and gas is  sucked  into  the 
crankcase. Simultaneously the crown of 
piston compresses the previous charge in 
cylinder head.
 (2) When the piston nears the top of 
the stroke this compressed charge is fired, 
the piston travels down, compressing the 
gas in the crankcase and delivering power 
to the crankshaft through which it is 
transmitted to the rear wheel.
 (3) The piston uncovers the exhaust 
ports, spent gas still at fairly high pressure 
rushes out into the silencers. Fractionally 
later the four transfer ports are uncovered, 
and the new charge from the crankcase is 
released into the cylinder. The design of 
the transfer ports, which enter the cylinder 
tangentially and inclined upwards, helps 
the new charge to clear the burned gas 
without mixing with it.
 (4) The piston rises, closing all ports, 
and a fresh cycle is begun.
OVERHAULING THE ENGINE
DECARBONISING
When an engine has run for a consider-
able mileage deposits of carbon are formed 
in the combustion space of the cylinder 
head, in the piston crown, under the 
piston rings and round the edges of the 
ports of the cylinder. This deposit affects 
the efficiency of the engine and must be 
removed. The engine need not be 
removed from the frame, but the following 
parts must be removed from the engine :—
 (1) Cylinder head. Using a screwdriver, 
remove the release valve control clip. 
(On some machines side shields are fitted ;
these must be removed.) Remove the four 
nuts securing the head in position after 
having taken out the sparking plug and 
the release valve complete. The piston 
crown and cylinder head can now be 
cleaned, but as it is more than likely that
the ports, ring grooves and silencing 
system also require decarbonising it will 
be necessary to remove the cylinder.
 (2) Remove the carburettor, exhaust 
manifolds and, where fitted, the long tail

pipe. The silencer can then be removed. 
Remove the four nuts securing the cylinder 
to the crankcase. The cylinder can now 
be pulled away, but do not turn it while 
doing this, as the piston ring joints may 
foul the ports. If the cylinder base joint 
washer is damaged, replace with a new 
one on reassembling the engine.
 (3) To clean the piston skirt and ring 
grooves it is advisable to remove the 
piston. Remove the circlip from one end 
of the gudgeon pin hole. The gudgeon pin 
is a floating fit, and will push out unless 
carbon deposit prevents this, when an 
extractor of the band type is useful. 
Remove the rings from the piston with 
great care as they are very brittle. Clean 
out the grooves and the carbon from the 
rings. A section of an old piston ring 
ground to a point makes an admirable 
tool. A knife will do, however. The 
piston, being of aluminium, is soft, so 
care should be taken not to remove any 
metal. Scrape the piston crown and the 
inside of the piston clean with a stick of 
solder cut or hammered to a chisel shape. 
The piston crown may be lightly polished. 
Wipe off all traces of polish and do not 
let it touch the piston skirt.
 (4) Clean out the various ports of the 
cylinder with a screwdriver or old knife. 
A bent tool will clean round corners more 
easily. The by-pass hole from the exhaust 
port to the top face of the cylinder must 
be cleared of carbon. Be careful not to 
damage the walls of the bore or the top 
face of the cylinder, which has to make a 
gaslight joint with the cylinder head.

FITTING  NEW RINGS
 When fitting new rings, test them in 
the cylinder by pressing them just inside 
the bore with the skirt of the piston. The 
gap between the ends should be 0.004in. to 
0.008in. Too small a gap will cause a 
seizure. An old ring with gap more than 
0.030in. needs replacing. Note the way the 
rings were originally fitted and replace 
exactly the same, dead square in the groove.

REMOVING  UNIT FROM FRAME
 The Villiers Junior de luxe is fitted to 
many of the leading motorised bicycles 
and the method of engine removal is 
apparent.

  How to mount the engine for attention
to the cylinder and flywheel.    One of
the two  exhaust manifolds  has been
removed

  Disconnect the rear chain and all controls 
and connections, including petrol pipes, 
throttle, choke and release valve control 
wires and lighting connection, take off the 
carburetter and the exhaust pipe stays if 
fitted. Engine and engine plates will 
usually come away together after removal 
of the securing bolts. NOTE.—A con-
nection is provided in the lighting cable, a 
short distance from the magneto ; un
screw this when removing engine from 
frame. Do not attempt to remove cable 
from inside magneto ; keep in position 
the rubber sleeve over the connection,
otherwise a short circuit may occur.
STRIPPING DOWN
Stand the engine upright by holding the 
bottom lug in a vice. Remove exhaust and 
inlet manifolds, and the nut holding the 
silencer to the clutch case. Remove the 
two slotted screws in the clutch cover in 
order to lift off the clutch bridge piece and 
lever complete, thus revealing the driving 
sprocket. When the engine is taken out 
of the vice it is advisable to tip out the 
clutch operating push-rod, which consists 
of three sections and a ball. The outer 
section will probably come away with the 
bridge piece.

MAGNETO
   To remove the magneto flywheel undo 
the nut, using a 7/16in. spanner and hammer. 
A special " hammertight" spanner is 
marketed by Villiers. NOTE.—This nut, 
right-hand thread, has a flange which draws 
the flywheel off as it is unscrewed. After 
the first turn or two it will be found to 
tighten because the flange is pulling against 
face of the jiywheel boss. Place a piece 
of wood against the face of the nut and 
give a sharp hammer blow. The flywheel 
will then be loosened on its taper and the 
nut can be turned with the fingers.
  Having removed the flywheel place a 
spanner or piece of iron across the pole 
shoes to prevent loss of magnetic flux.
  Releasing four screws in the centre now 
enables the armature plate, magneto com-
ponents and contact breaker to be taken 
off as a unit. If these screws are obdurate, 
tap the end of the screwdriver sharply 
with a hammer before turning.
  The contact breaker points are accessible 
without removing the flywheel, by remov-
ing the three small screws which secure 
the large disc in front of flywheel.

REMOVING CYLINDER
This operation has already been detailed 
under " decarbonising."



 Undoing the flywheel nut with a
  special "hammertight" spanner

  CLUTCH
Lay the engine on its side with  the  pro-
truding shafts uppermost and remove the 
seven  nuts  and  washers  holding  clutch 
cover.   Remove   the  sprocket   from 
the clutch shaft by unscrewing hexagon nut 
(right-hand  thread)  anti-clockwise.  On 
later models there is a lock washer under-
neath  the  nut  which  must  first  of all be 
flattened to  allow  the  nut  to  turn.  Pull 
off the sprocket,  using  a  claw  type  ex-
tractor, and remove the key from the shaft. 
The clutch cover can now be removed by 
tapping gently the  ears  provided  on  the 
cover,  both  ends  at  the  same  time.
  With the cover off the primary  chain, 
engine sprocket and clutch sprockets are 
revealed. On the other side  of  the  case 
is a small nameplate held in position by 
four slotted screws. When this cover is 
removed the clutch shaft can be drifted 
through from that side, bringing away the 
complete  clutch, which  will  drop  apart 
when released from the chain. If the chain 
is very tight it may be necessary to remove 
the mainshaft sprocket, but after some 
service the chain is usually slack enough 
to dispense with this, as it will ride off 
when the clutch sprocket is tilted.
  To remove the mainshaft sprocket first 
flatten the lock washer fitted under the 
nut, wedge the connecting rod in the 
crankcase to prevent turning and undo 
the nut with an 11/16in. box spanner. The 
sprocket will now come off leaving the 
key in position.
CLUTCH REASSEMBLY
  The clutch must be assembled as a unit 
before replacing it in the chain case. Hold 
the shaft with the threaded end upwards 
and slide on the short spring sleeve (1), 
spring (2), long sleeve (3) and outer clutch 
plate (thick plate with six holes) (4), shiny 
side uppermost. Press down the plate and 
insert the flat cotter (5) in the slot in the 
shaft, with the ears towards the top. Then 
assemble the cork insert plate (6) with 
the driving tangs upwards, the centre plate 
(dished) (7) with the bulge downwards, 
followed by the driving sprocket (8), 
either way on, the solid outer plate (9) and 
the ball race.
  In order to engage the outer plate with 
the splines on the shaft the whole assembly 
must be compressed and held until the

  Special jig for compressing the clutch
spring.    If this  is  not  available the
procedure outlined in the text is quite
effective

clutch case cover has been secured in place. 
A special jig is used at the factory, and 
the Villiers Company can supply a simple 
jig, but in the absence of this aid the 
simplest way is to proceed as follows :—
 Obtain a tubular distance piece 11/16in. 
long and not more than 3/4in. in diameter 
which will fit easily on the shaft, and 
assemble this over the ball race. Then 
take a piece of steel rod 3/16in. in diameter 
and about 3in. long. Insert this in the 
hole down the middle of the shaft and 
thread the nut over it. Lift the whole 
assembly carefully and place it endways 
between the jaws of a vice. Tighten the 
vice so that the push-rod compresses the 
clutch spring. This will enable the nut to 
be screwed on to the end of the shaft and 
tightened gently.
  Fit the chain round the clutch sprocket 
and replace the assembly in the chain case, 
passing the chain round the engine 
sprocket before pushing the clutch home 
in the ball race at the back. Replace the
name plate. The cover can now be fitted, 
and it will be found that the nut on the 
end of the clutch shaft protrudes through 
the hole. If the cover is heated slightly in 
boiling water before fitting it will drop 
over the ball race quite easily. Do not 
omit the thin jointing washer. When the 
nuts holding the cover have been tightened 
down the nut can be removed from the 
shaft and the distance piece replaced by 
the sprocket, after the Woodruffe key has 
been inserted. Tighten the nut fully and 
turn up the lock washer (if fitted). Then 
with the unit lying on the bench insert 
the sections of the clutch operating push 
rod, long section first, then ball, short

 

  section and medium section, the last fitting 
in a bush in the bridge piece, which should 
be attached last.
When the unit is replaced in the frame 
check the adjustment of the clutch control 
wire, and adjust with the grub screw and 
lock nut on the lever .on the bridge-piece 
so that there is slight play on the lever, 
ensuring that the clutch is fully engaged 
and that the push-rod is not in compression 
under normal running Conditions.

BIG END AND CRANK CASE
  When the clutch is down the only 
material part left to strip is the crank 
assembly. Remove the right-hand crank-
case cover and take out the key in the 
protruding end of the shaft. Using a soft 
hammer, tap out the shaft, connecting rod 
and crank assembly complete.
RENEWAL OF BEARINGS

  All bearings can be renewed, but a 
certain amount of skill and care is needed, 
and renewals should not be undertaken 
unless they are thoroughly understood.
  The little end of the connecting rod has 
a bronze bush which floats in the rod and 
on the gudgeon pin. Three 3/16in. oil holes 
are drilled round the bush. The gudgeon 
pin hole in the piston is bushed, the bushes 
having grooves for the locating circlips.
  Renewal of the big end bearing calls for 
a press. First drive out the rivet through 
the crankpin. This releases the large 
washer which retains the rod and rollers 
on the pin. There are eight steel and 
eight bronze rollers. The pin can be 
pressed out from the back.
  New pins are supplied 0.001in. oversize 
on the part which is pressed into the 
crank web. An oversize pin must always 
be used as once a pin has been pressed in 
and extracted, the hole is slightly enlarged.
  Press in the new pin, using a pilot to 
ensure that it goes in absolutely square. 
Assemble the rod and rollers (putting the 
bronze rollers between the steel ones) with 
a generous smear of engine oil, and rivet 
the washer on, punching the end of the 
rivet into the slots in the washer.
  Should the main ball bearing require 
renewal remove the retaining plate screwed 
to the inside of the crankcase and tap out 
the bearing from the other side. Note the
 

 
crankcase sealing gland which is retained by 
the ball race and consists of a flat steel 
spring washer, slightly dished and slotted, 
and a bronze bush. It should be noted 
that the bush is a very close running fit 
on the crank shaft, being internally ground, 
but is a loose fit, with about 0.030in. 
clearance, in the crankcase. The seal is 
actually made by the flange of the bush 
being pressed against the face of the crank-
case by the spring washer.
  When assembling a new bearing insert 
the bush and place the spring washer over 
it with the dish towards the bush. Press 
in the bearing and replace the retaining 
plate.
  The hole in the chaincase cover through 
which the crankshaft passes is sealed with 
a bronze bush pressed into the cover out-
side the ball-bearing. As the load is taken 
by the ball-bearing this bush should never 
need renewing.
  When reassembling the engine note the 
paper washer which goes between the two 
halves of the crankcase and the 1/32in thick 
jointing washer between the base of the 
cylinder and the crankcase. There is no 
gasket of any kind between the cylinder 
and head. Jointing compound should not 
be used on any of these joints. Tighten all 
flange nuts a little at a time, going from 
side to side or, in the case of the cylinder 
base and head, from corner to corner, 
tightening " opposite " nuts in sequence.
  If the cylinder becomes worn or badly 
scored it can be rebored, but a very high 
finish is desirable. Replacement pistons 
are available in 0.015in. and 0.030in. over-
sizes. When reboring measure the piston 
skirt accurately and allow 0.004in.-0.005in. 
for skirt clearance on the diameter.

CARBURETTOR
  Reference has been made to the gas 
filling the crankcase by way of the inlet 
port, and it may be as well to explain 
how this gas is formed, for the benefit of 
those unacquainted with petrol engines.
  The carburettor is responsible for this 
function and on motor-assisted bicycles 
this Villiers instrument contains three main 
parts—the float chamber, the throttle and 
the mixture control needle.
  Petrol enters the float chamber from 
the petrol tank by way of an orifice (E)
which can be sealed by a tapered needle

valve (F). The air-filled brass float rises 
when the petrol flows in and when it 
reaches a certain height forces the needle 
valve on to its seat and stops any further 
flow of fuel until the engine's demand for 
fuel has again lowered the level. Petrol 
passes from the float chamber through a 
jet in which slides a long needle with a 
fine taper (D). This needle is connected

with the throttle (C), which is cylindrical 
in shape and slides up and down across 
the passage through which air passes to 
the engine.
  When the engine is not required to 
produce much power the throttle is nearly 
in its lowest position and the mixture 
control needle fills the jet orifice. Thus 
only a small amount of air is allowed to 

pass and an equally small amount of petrol 
is sucked from the jet to mix with the fast 
moving air-stream and thus be vaporised. 
  When the rider needs more power he 
opens his throttle lever which raises the 
throttle plunger and allows more air to 
pass to the engine. At the same time the 
throttle plunger lifts the mixture control 
needle and allows more petrol to emerge 
from the jet. Thus the proportion of 
petrol to air is kept constant at all positions 
of the throttle.

TRACING TROUBLE
  Troubles are usually traceable to dirt on 
the float chamber needle, causing creep or 
flooding, or dirt in the filter, loose top cap, 
punctured float, air leak through fixings 
becoming slack, air-lock in tank (cure by 
removing petrol cap and blowing into the 
tank), too much or wrong kind of oil in 
petroil mixture, choked fuel pipe or tap. 
The strangler, of course, is only used to 
give rich mixture on starting. Release 
when the engine is warm or starts to  
" four-stroke " when running or idling.

DISMANTLING
It is best to detach the carburettor from
  

the engine. Unscrew the top ring A and 
lift out the throttle slide C. Invert the 
instrument and unscrew the bottom nut B. 
Take off the fibre washer, lift off the cup C 
and float H, undo the small grub screw 
in the side of the body. Push out the 
centre-piece and jet J, noting the position 
of the fibre washer under the head. The 
lever controlling the fuel needle can now 
be pushed aside and the needle removed. 
A screw-on type of air filter is fitted, the 
gauze of which should be cleaned at 
intervals by dipping in petrol.

ADJUSTMENT
To adjust the needle first of all remove 
the throttle by unscrewing the top ring. 
At the head of the throttle there is a small 
slotted screw K. Turning this in a clock-
wise direction lowers the needle and will 
give a weaker setting. Turning in an 
anti-clockwise direction will give a richer 
setting. For adjustment give approxi-
mately half a turn at a time until it is 
found to be correct. As a starting point 
if the adjustment has been entirely lost, 
the top of the screw should be about two

threads  or  1/16in.   below   the  top  of the 
throttle.
  If the float cup has to be removed at 
any time for cleaning, when re-assembling 
do not use too much force in tightening 
the bottom nut. Periodically see that the 
gauze in the petrol pipe " banjo " con-
nection L is free from dirt, or the petrol 
will not flow freely.

IGNITION
To fire the charge of petrol gas when 
it is compressed in the cylinder an electric 
spark is required. This is produced by 
a magneto, an instrument which produces 
electricity at a very high voltage.
  To produce that voltage a rotating 
magnet builds up a current in a coil of 
wire. At the instant when the spark is 
required that current is prevented from 
flowing by the separation of two contacts 
operated by a cam on the crankshaft. A 
sudden surge of current is caused in the 
primary coil, and by a process known as 
induction this surge causes another current 
of a very much greater voltage to flow in 
a secondary coil which surrounds the first 
or " primary " coil.
  From the secondary coil an insulated 
cable takes the momentary high voltage to 
the sparking plug, which is screwed into 
the top of the cylinder head and consists 
of a central rod to which the high-tension 
cable is connected. This central rod or 
electrode is insulated from the main body 
of the plug, at the innermost end of which 
is a small projection almost touching the 
central electrode. Electricity must always 
flow in a circle, and here the high tension 
current flows through the secondary coil, 
along the high-tension cable, down the 
central electrode, whence it jumps to the 
body of the plug over the small gap pro-
vided and returns to the secondary coil by 
way of the cylinder and crankcase. When 
it jumps the small gap it forms a spark 
and thus ignites the mixture.
MAGNETO AND TIMING
  The contact points in the magneto are 
enclosed in a circular brass case. (On 
early models this has a cap attached by a

 
Access to the magneto contact-breaker is
gained  by  removing   the flywheel  cover
plate.     The breaker unit is visible inside
the white circle

spring, which can be pushed aside.) The 
points should break or open when the 
piston is 1/4in. from the end of the outward 
stroke. The position of the piston can be 
checked by a pencil through the sparking 
plug hole in the head. If the timing is not 
correct loosen the flywheel as described 
under " Magneto," turn the engine anti-
clockwise until the piston is at the extreme 
end of its stroke nearest to the cylinder 
head. Then position the flywheel by hand 
with the mark on the rim in line with the 
arrow stamped on the armature plate near 
the high-tension terminal, taking care that 
the crankshaft does not move. Hold the 
flywheel firmly in this position and lock 
up the centre nut with the hammertight 
spanner.
  Check the contact-breaker points after 
the first 150/200 miles. Turn the flywheel 
to lift the rocker arm to its highest position. 
Undo the locknut (the lower one) and 
turn the other nut until the faces of the 
points are 1/64in. (.015in.) apart, using the

feeler gauge supplied with the magneto, 
then  re-tighten  the  locknut. Never file 
the points. Clean if necessary with a non-
woolly rag dipped in petrol.
SPARKING PLUG
  The  gap  between the side and  central 
electrodes should  be 0.020in. In use  the 
points of these electrodes get burnt, thus 
causing the gap to  widen . Close  up  the 
gap by bending the side electrode. Never 
bend the central electrode .  If  there  is 
plenty of fuel in the  carburettor  and  the 
engine fails to start, remove  the  sparking 
plug and examine the points,  which  may 
be   bridged   by   a  film  of  oil.    Clean 
thoroughly in petrol.
LUBRICATION
  Petroil lubrication—a mixture of 1/2 pint 
of the recommended grade of oil with each 
gallon of petrol—is employed. It  is  of  the 
greatest   importance   that   this    mixture 
should be made in its correct ratio  of  one 
part oil to 16 parts petrol, and  it  must  be 
well shaken in a  tin  to  ensure  thorough 
mixing  before   pouring   into    the    tank. 
Never pour the oil and  petrol  separately 
into  the  tank  and  trust  to  vibration  for 
mixing them.

 View of the Villiers carburettor, showing,
in the white circle, the grub screw which
must be taken out before the centrepiece
and jet " J " can be removed

  The chaincase  is  lubricated  separately 
through a filler plug, a  different  grade  of 
lubricant to that used for the engine being 
recommended (see " Tabulated Data " on 
p. 2).  It  is  important  that  the  level  of 
oil should be maintained.