Automatic Scooter Engine
The CYLINDER/PISTON
The cylinder, cylinder-head and piston are some of the
most important components in the engine. The upward
stroke of the piston draws in the fuel/air mixture from the
carburettor and the downward stroke of the piston
transfers this mixture up to the cylinder head, the power
created by the explosion in the cylinder head (combustion
chamber) is transferred to the engine via the crankshaft
and connecting rod. The design, material and shape of
these components are critical to achieve the desired
performance. The upper part of the piston with the help of
the piston rings has to provide a gas tight seal within the
cylinder. The position of the piston and the size / shape
and position of the cylinder ports, control the flow of gas
through the cylinder.
The CARBURETTOR
The carburettor is the control for the
engine. It feeds the engine with a
mixture of air and petrol in a
controlled volume that determines the
speed, acceleration and deceleration
of the engine.
The carburettor is controlled by a slide
connected to the throttle cable from
the handlebar twist grip which adjusts
the volume of air drawn into the
engine. A system of fixed holes (jets)
and, adjustable needles control the
volume of fuel (petrol) to match the
volume of air. The ratio of fuel and air
the carburettor delivers can be
changed by selecting different jets
and adjusting the position of the
needles. Sometimes but not very
often, the manufacturers restrict the
speed of the scooter for certain
countries by fitting a reducing washer
to the entrance of the carburettor.
Adjustments
Although the carburettor is set by the manufacturer, it is sometimes necessary to make
adjustments to match the characteristics of the engine. These are:
1- Tick-over, this controls the speed of the engine when the twist grip is closed to stop the
engine from stalling.
2- Mixture screw, this controls the volume of fuel that enters the engine when accelerating from
a low engine speed.
3- The height of the float and float needle controls the amount of fuel available in the the
carburettor. If the float needle becomes worn it may cause the carburettor to leak and overflow.
4- Height of slide needle, controls how quickly the needle is lifted from the main jet, to allow the
fuel to pass through the jet.
5- Size of the main jet, selecting a main jet with a different size hole will effect the volume of fuel
it will pass.
For example when a new exhaust is installed or a custom air filter it is likely that a higher ratio of
fuel to air will be necessary and therefore a main jet with a larger hole will be required.
THE EXHAUST
The exhaust has a very important role in the performance of a two stroke engine. The
design of the exhaust is critical to achieve the maximum power. Following combustion in
the cylinder head the burnt gases are discharged through the exhaust port and they expand
into the shaped chamber of the exhaust. The shape, size and length of this chamber are
critical to the efficiency of the system.
At the end of the chamber, there is a tapered outlet into the silencer. This tapered outlet creates
a reverse pulse, back to the cylinder which is timed to reach the exhaust port at the moment the
piston passes it and closes it off. This reverse pulse helps to ensure that none of the incoming
fuel/air mixture escapes and the maximum is retained in the cylinder for combustion.
It is common for the scooter manufacturers to restrict the speed of the scooter for certain
countries by tack welding a washer or small restrictor to the entrance of the exhaust or at the
exit to the silencer, these can usually quite easily be removed. Sometimes another way for the
manufacturers to restrict the scooters speed is for them weld an extra piece of pipe some 15 or
20 cm long to the outside of the exhaust lead in pipe, again a performance increase will be
experienced if this is removed, and weld over the hole it leaves.
Each exhaust is designed and tuned to achieve maximum performance with a specific engine.
1 - the exhaust evacuates the burnt combustion gases
2 - the exhaust reduces the sound from the combustion chamber
3 - the exhaust is tuned to improve the efficiency of the port timing.
The Variator
The variator is like a continuously variable drive system to give the optimum gear ratio
between the engine and the rear wheel. This enables the engine to work at the best speed in
comparison to the rear speed.
The variator works upon the ratio of the diameter of the front and rear drive belt pulleys.
Both front and rear pulleys are an assembly of moving plates. When viewed from the side, one
plate is convex and the other is concave. At low engine speed, the plates of the front pulley
are pushed apart by the tension of the belt as it forces itself between the plates. At the same
time, the plates in the rear pulley are pushed together by the spring beneath the automatic
clutch. Behind the moving plate of the front pulley, there are weight rollers, these are thrown
towards the outside edge of the variator by centrifugal force, as the rotation speed of the
engine / variator increases. When the weight rollers move towards the outside of the variator,
the inside plate is forced together with the outer plate and this moves the drive belt towards
the outside of the pulley. This effectively makes the diameter of the front pulley larger. As the
belt moves out on the front pulley, the tension in the belt is increased, which overcomes the
spring pressure holding the two plates together in the rear pulley. The belt moves towards the
center of the rear pulley, which effectively makes the diameter of the rear pulley smaller. In
this way, the ratio between the two pulleys is changed and therefore, the speed ratio between
the engine and rear wheel is changed. It is common for the scooter manufacturers to restrict
the speed of the scooter by fitting a spacer washer under the variator. This restricts the
amount the variator is allowed to open. if removed the belt is allowed to go further out into
the pulley.
FINAL DRIVE ASSEMBLY
The final drive assembly drives the rear
wheel; this is a combination of shafts and
gears that determine the speed of the wheel
in relation to the speed of the engine. In
standard form the ratio is set to allow the
engine to turn at an optimum speed according
to the wheel speed. If the performance of the
engine is improved it is possible to change the
gears in the final drive assembly for a new
ratio.
Aprilia sr 50cc
The Differences
Developed from the air-cooled motors with horizontal cylinders, these
engines have had a number of changes over the years. The major changes
concern the transmission parts: the rear pulley, the clutch diameter and the
rear pulley diameter changed around 1996/7, but apart from cosmetic
differences and the use of a disc brake on the rear, the motors are
basically the same.
Derestricting
Derestriction usually involves removing the restricter from the front variator,
removing the washer from the exhaust and cutting the front pipe 90° elbow
off and rewelding. When derestricted by these methods the top speed is
around 50 to 55mph on the speedo.
Exhaust
The usual way of improving performance after derestricting is to fit a better
exhaust. The LeoVinci Handmades and the Giannelli NRX exhausts give a
good boost in power, but with a narrow powerband they can give the
transmission problems if roller weights are not spot-on for the individual
motor.
The exhausts with wider more flexible powerbands are the Giannelli Sport
Cee and the LeoVinci SP3. These give good performance but at lower
RPM than the NRX and Handmade pipes. They are better suited to the
70cc kits for road use where a gear kit is to be fitted. Because the RPM is
lower with these exhausts potential engine wear is reduced.
Further Improvements
Like most Yamaha automatic motors, the transmission allows large
variations in RPM, so using aftermarket springs, variators, and a torque
driver pulley will benefit performance.
The use of a 70cc cylinder kit with a larger carb and a gear kit will give
another boost in performance, the increase depending on what other parts
are fitted - around 8 to 9bhp is achieved with the basic performance parts
and around 10 to 11bhp with the aluminium MHR kit from Malossi and the
W-port aluminium kit from Polini. These higher performance kits will benefit
from the race spec transmissions that Malossi make under the wide-range
transmission format, which must be fitted and used as complete sets.
The cast-iron basic cylinder kits such as those made by DR, Top
Performance, Malossi and Polini can use 17.5mm, 21mm or 22mm carbs,
but the more expensive larger carbs only give a small power increase over
the 17.5mm when used with the cast-iron cylinders.
When aluminium cylinders are used the larger carbs become more
beneficial. Any of the cylinder kits will run on any of the carbs, including the
standard one, but like everything else it depends on performance required
and budget.
Ignition
Standard ignition retards above 10,000rpm, so some combinations of parts
require an uprated CDI. Use either a straight replacement without a rev
limiter or one with retarding ignition timing to suit performance motors with
higher compression.
Crank
If an uprated crank is desired then race cranks are available to use the
standard 10mm small end or 12mm small end, swapping the piston in the
kit to suit. The 12mm small end is the stronger and better option, especially
with aluminium kits and high-revving exhausts.
Reed Petals and Blocks
Uprated carbon fibre petals are available to fit the original reed block or a
complete Malossi or DR reed block assembly fitted with carbon petals.
There are carbon fibre reed petals to fit the standard reed block that will
improve performance and there are replacement reed valve blocks with
carbon petals to replace the original. Our experience of replacing the block
has shown that it only becomes beneficial with high-revving motors. When
thinner 0.25mm or 0.3mm petals are used the better cushioning effect of
these blocks increases the life of the petals. In general, the thinner the
petals the more susceptible to fraying edges they are. 0.3mm for road
engines is a good compromise.
Airbox
All the engine units have rubber inlet tubes feeding into the airbox to
reduce noise. Removing this trumpet will allow more air to enter the airbox
and into the engine more freely, but it’s not just a matter of removing it. The
carb main jet must be increased considerably and the needle lifted to
provide more fuel.
For example, a 17.5mm carb with a 70cc kit and exhaust on a standard
airbox requires the main jet in the 75-80 range, but with the rubber inlet
removed the main jet is in the 88-90 range. With a 22mm carb and the inlet
rubber fitted the main jet is 75, but without the inlet rubber it is 88-90.
Failure to use the correct main jet will cause damage to the motor.
Gearing
The standard motors are geared fairly high. With the choice of parts giving
different operating RPM the overall gearing can sometimes be left the
same or up-geared to increase the top speed.
Between DR, Polini and Malossi we have a range of gearing available from
+10% to +60%. It is always worthwhile trying to match the gearing to the
power output and motor configuration correctly. Being geared lower wastes
speed and increases wear at top speed. Gearing too high gives the clutch
and drive belt a harder time than necessary when moving off.
Contrary to popular belief, once the motor is operating on the pulleys at
peak RPM the overall gearing used does not affect the acceleration. The
transmission just uses a different part of the pulley’s diameters. We
normally use the manufacturer’s recommendation for gearing but
occasionally better results are obtained with a different set-up.
Yamaha/Minarelli Style Motors
(As used in some Aprilia, Italjet, Malaguti, Beta, Benelli,
MBK, Motron, Yamaha scooters etc.)
The Yamaha engines are in three types, two air-cooled and one watercooled, all made under license by Minarelli in Italy and supplied to a
number of manufacturers.
The oldest was originally used in the Yamaha BWS. It has an upright
cylinder and the carb feeding through a reed valve into the rear of the
cylinder barrel.
The later engine unit was originally used in the Yamaha Jog, which has a
similar layout to the BWS motor, but has a horizontal not vertical cylinder
with the carb feeding into the crankcase not the cylinder. This motor was
originally air-cooled and for the last few years has been available in watercooled form.
Up until recently the motors were all drum brake at the rear with either 10"
or 13" wheels. Now the latest models are available with a disc brake on the
rear. These water and air cooled motors had an update in 1995/96 when
the transmission clutch and rear pulley system was revised.
All the restricted motors are restricted by a spacer between the front
pulleys, restricters in the exhausts (not always) and a rev limiter built into
the CDI unit. The rev limiter can be disconnected on early motors by
disconnecting one wire. On later motors (’96 onwards) there was an
increase in the number of manufacturers using the latest CDI units, which
must be replaced to remove the rev limiter. These replacement CDI units
can have standard ignition timing or variable ignition timing with no rev
limiter (which is useful when higher states of tune are used).
One of the problems with all these engine units is that the RPM varies
greatly as the pulleys change diameter, and that the standard transmission
spring will not cope with 70cc power outputs without obtaining belt slip at
low speeds. The easiest way to help both these problems is to use an
aftermarket variator and rear pulley spring to enhance transmission
performance. The rear half of the rear pulley can be changed so that RPM
change is reduced to a minimum as the pulleys change diameter. This rear
pulley half is referred to as a torque driver pulley.
The higher state of tune aluminium MHR or race cylinders with race
expansions can beneficially use the race/fast-road transmission set-up,
which uses a matched rear pulley assembly and variator and a special
drive belt. The only extra part required is a clutch assembly to obtain a
complete transmission system.
If an exhaust is used with a cylinder that between them generates a wider
powerband then the only transmission part required is an uprated rear
spring so that belt slip is eliminated.
The most popular aftermarket exhausts are those supplied by LeoVinci in
several different styles:
Sito Plus
Standard-looking box with enhanced performance. Does not usually
require any transmission modifications on a standard bike. It is a useful
way of obtaining a bit of extra power when replacing a standard box.
SP3
This is a standard style box but it increases performance by around 20%
on air-cooled and 10% on water-cooled motors. It is supplied with rollers to
fit the standard transmission parts.
Navigator
Latest style box from LeoVinci. Gives more performance than the SP3 and
is supplied with rollers and sometimes clutch springs. Not as good as the
handmade exhaust.
Handmade Exhaust
Gives 35/40% power increase on air-cooled and around 25% on watercooled motors. This comes with rollers and clutch springs so that the
transmission works in the right range.
These exhausts are all very reliable, improving both top speed and
acceleration.
Malossi do two exhausts. One is a road exhaust called Wild Lion which is
not normally used as the LeoVinci gives the same performance at a
cheaper price. The other is a race expansion which is extremely expensive
but will not work with the centre stands and is designed to work with MHR
replica and MHR race cylinders.
Polini do a range of exhausts but the basic two systems are covered by
LeoVinci. The race box is cheaper than the Malossi version but again is
designed for their cylinders and will not clear a stand. They do a road
expansion chamber that suits the 70cc kits very well, the cast-iron and
aluminium road units from both Malossi and Polini. It is slightly dearer than
the LeoVinci carbon fibre handmade, but works at a lower RPM range than
the LeoVinci, which means it suits the MHR and aluminium kits better when
used on the road.
The clutches are available in two and three shoe types. Aftermarket springs
are available to alter the biting point to tune the transmission. On higher
states of tune where enough slip cannot be generated by the springs,
aftermarket clutches are available that have a choice of springs and lighter
shoes to give more adjustment. The ultimate clutches are Polini speed
clutches and Malossi delta clutches. These have a choice of springs and
spring positions and movable weights to allow the clutch to work with any
motor from standard to full race.
Cylinder kits are available in several formats, basically cast-iron with or
without cylinder heads, road aluminium with cylinder head and race
aluminium with cylinder head.
The race and MHR motor can run at RPMs that will increase stress on the
crankshaft so that it becomes advisory to change them. Race crankshafts
are available from Malossi and Polini that are good for 16,000rpm, well
above what road motors should be built to run at - so are extremely
reliable. They are available with a small end to take 10mm standard
gudgeon pin size or 12mm to suit different pistons that are available to fit
the cylinder kits. The12mm gudgeon pin further improves reliability on high
states of tune. If you are going to use MHR or MHR-Replica type cylinders
and have a performance crankshaft that uses the 12mm type crank then
get the cylinder with the 12mm piston kit.
The engine units use autolube but the pump is not connected to the
throttle, so the oil delivery is dependable on engine RPM only. When using
tuned motors you must use fully synthetic two-stroke oil to avoid problems
with lubrication, and if MHR or race cylinders are used either add extra oil
in the petrol or remove the autolube and use oil at 3% with the petrol.
Carbs come in a lot of different sizes; the standard carb is 12mm. For road
use 17.5mm is large enough, but MHR or aluminium cylinder kits with
expansion chambers will benefit from larger carbs - 22mm is popular.
Some motors have auto chokes and some cable chokes - carbs can be
supplied with either. There are also 19 and 21mm carbs listed by Malossi
and Polini that are cable choke only.
To raise the overall gearing there are several types of gear kit available,
with different numbers of teeth to suit the various models. The best option
is to use the gear kit recommended by the kit manufacturer to obtain a
balance between power and gearing.
It is becoming more popular to use these motors for racing in Italy, so there
are special cylinders and crankshafts for motocross and race use that must
use the appropriate crankshaft, exhaust and transmission parts. The
operating RPM is between 13,000 and 14,000rpm, and although very
powerful the wear rate for road use is not acceptable. The life of some
components in race use is as little as 10 hours and a full engine rebuilt of
crankshaft and bearings is required every 25 hours of use to ensure
reliability!
At the normal sub-10,000rpm of the road motor, reliability is excellent if
good two-stroke oil is used and the carburation is set up correctly to avoid
overheating.
As with all tuned motors, you need to decide what end result you require
and select the appropriate components to give you this. For example, if
only a limited increase in performance is required, this could be achieved
by using an exhaust on a standard cylinder or a road cylinder kit with the
standard exhaust. If you need large increases in top speed you have to upgear the transmission with a suitable gear kit and then find the power to
obtain this top speed.
If you need any advice on what mix of bits and pieces you require to obtain
the desired level of performance then please contact the shop.
New Wide-Range Pulley System for Race Engines
Malossi Transmission System
for Aprilia-Benelli-Italjet-Malaguti-MBK-Yamaha
When MHR and MHR Replica cylinders are used with race or race style
exhausts systems they create a narrow power band that the original pulley
systems cannot cope with. Normal aftermarket pulleys use a standard rear
pulley half and drive belt size. The latest pulley system uses a wider belt
and larger diameter pulleys to increase the operating range of the pulley
system. Together with revised pulley angles, it allows even highly tuned
race engines to stay in the working power range. However, although the
parts are similar to or the same as some previously listed components,
they are not compatible with standard components and must be used as a
complete system.
Relying on its leadership in the field of automatic scooter transmissions,
Malossi has extended its technological skills to the new automatic
transmission for competition scooters with Minarelli and Yamaha engines.
This new transmission has been designed and tested on engines ready for
scooter cross and racing competition, demonstrating excellent reliability
and extremely high levels of performance. This new system hinges on the
new rear torque driver Rear Pulley System that has two pulleys with a
much larger diameter than the original and the very latest technological
specifications.
The larger diameter of these pulleys allows one to have a greater velocityratio range compared to the original pulleys that have a notably smaller
external diameter.
Malossi engineers have also taken particular care in realising this pulley
right down to the smallest detail, using special materials and a particular
type of thermochemical treatment that ensures exceptional surface
hardness and therefore excellent wear and abrasion resistance. In order to
fully exploit the range of the new torque driver unit and the power ratings
currently offered by competition machines, a new Malossi notched Kevlar
Belt has been introduced with larger cross section and revised length to
ensure maximum performance under all running conditions. To complete
the renewal of the transmission system and take full advantage of its
exceptional specifications, the Multivar variable-speed drive has also
been upgraded by fitting it with new tracks for the rollers and a new hub.
This new variable-speed drive represents the evolution of the previous
generations, the result of many years experience on the race circuit and
the currently available CAD-CAM technology that allows top-level
specifications to be obtained.
The combination of these new components creates a perfect cocktail that
will allow you to savour the taste of victory.
The new Malossi system must be fitted AS A WHOLE
Available for:
Aprilia Sr 94, Sr Replica, Sr Netscaper, Sr Stealth, Sr Racing, Sr WWW;
Benelli 491 Gt, 491 Sport, K2 (air cooled and liquid cooled), Italjet Dragster;
Malaguti F12 Phantom, F15 Firefox;
Mbk Booster, Booster Next Gen., Flipper, Nitro, Ovetto, Rocket;
Yamaha Aerox, Bws, Bws Next Gen., Neo's, Spy, Zuma
Variable-Speed Drive 51 9725
Kevlar Belt
61 9713
Rear Pulley System
61 9733
Peugeot Air/Water Cooled 50cc Scooters
Peugeot Elyseo, Trekker, Vivacity, Speedfight, Buxy & Squab
Scooters
These motors are basically identical except for water-cooled cylinders and
water pump driven from the flywheel right-hand side of the motor, replacing
the fan on the air-cooled engine and approximately 10% higher gearing in
the water-cooled motors.
Derestricting
Despite the many differences in the bodywork and exhaust styling, they are
all restricted in the same way :
1. By the exhaust being fitted with a resonant tube (a blind pipe
attached to the front pipe) that should be cut off and welded up. This
improves power and rev-on.
2. By a gearing restrictor in the variator body. This takes the form of a
large diameter plate next to the roller drive plate in the rear of the
front pulley. It just requires removal of the variator body cover and
removal of the plate and reassembly.
If these derestrictions are done correctly the differences shown in the dyno
curves will be apparent.
Fitting a 70cc Cylinder Kit
Dyno graph showing the different stages carried out
on the same Peugeot 50cc scooter
We would normally fit a slightly larger main jet, approximately size 76/78 if
size 72/74 was originally fitted.
To improve power output further the next stage is to fit a 70cc cylinder kit.
These are available very cheaply for the air-cooled and water-cooled
motors.
The DR air-cooled cylinder kit comes complete with a size 80 main jet for
the carb and fits using the original head. Power is improved as shown on
the dyno graph.
Exhausts
Compared to the standard exhausts most aftermarket exhausts will give
more power, but not necessarily more RPM. So you can have the effect of
obtaining more acceleration and less top speed compared to a derestricted
motor. This is not such a bad result, as fitting a gear kit will increase the top
speed whilst engine revs stay lower than with those exhausts that obtain
extra speed for extra high RPM. Also, the Peugeot motors are not that
smooth when revved very highly, and we prefer to obtain the power at
lower engine RPM and then fit a gear kit if higher top speed is wanted.
If a 70cc cylinder kit is used then a lower revving exhaust and a gear kit will
make vibration less apparent, which is preferable.
For maximum power on 50’s use a LeoVinci Handmade ZX or Giannelli
NRX expansion exhaust. For mid-range power use a Giannelli Sport Cee
exhaust or a Tecnigas expansion exhaust.
Carburettor
When more power is required then a larger carb and better reed block is
necessary. The Malossi carb kit comes with adapters to fit to the original
manifold and is 19mm in size. It also converts to a manual choke.
The transmission will not cope without uprating. The belt slip becomes
more noticeable with the extra power, so the fitting of an aftermarket front
pulley and Kevlar belt, preferably with the torque driver rear pulley half, will
give improved engine response and better acceleration through the midrange by reducing RPM variation in the engine.
The water-cooled motors are approached the same way as the air-cooled.
All of the motors have a very poor inlet manifold/reed block design and the
power outputs achieved are not as good as with other engine makes. It will
always be worthwhile to clean out any casting flaws and to match the inlet
port carefully to improve air flow in this area. The use of the DR reed cage
assembly which changes from 3 small to 2 large reed petals is
recommended for all states of tune.
Peugeot Speedfight, Trekker & Elyseo 100 Tuning Guide
Parts for these machines are slowly becoming available from European
manufacturers.
The biggest single improvement you can make is by fitting one of the exhaust
systems. To see the difference look at the Dynojet graph below comparing the
standard exhaust with an aftermarket Giannelli Freeway exhaust.
The larger 21mm carb kit from Malossi improves this again, but the major change
is the exhaust.
There are also some improved transmission parts available that extend the
range. We will provide Dynojet graphs showing the use of these parts as soon as
we have them available.
With the exhaust fitted the power output is so much improved that the 100 is
even more under-geared than standard and can usefully do with gearing 20-25%
higher. As soon as we have chance to check out the internals we will sort one out
but I’m sure the Italian aftermarket parts manufacturers will have something
available soon.
Gilera DNA 50cc Scooters
Introduction
The initial assumption with the Gilera DNA 50 scooter is that it is just a
water-cooled version of the Hiper2 motor in the Zip 2000 Kat. But the 14
inch rear wheel, the crankcase length and the gear box layout are not the
same as used in earlier Piaggio 50cc automatics.
The gear transfer box in the DNA motor is now accessed by unbolting the
case area from behind the rear wheel. (Previously this was part of the main
crankcase and was accessed through a cover under the rear transmission
pulley.)
The DNA is restricted in the usual way for Piaggio and Gilera scooters - a
washer in the front pulley (actually two) and a resonant pipe on the
exhaust, the effects of which can be seen in the dyno curve below. A main
jet increase is needed when derestricting.
The problem with any tuning using the catalysed standard exhaust is that
the exhaust gas analyser machines that measure the CO content are not
accurate, which makes it difficult to confirm what jetting changes are
required when engine modifications are made with the standard exhaust.
So, when derestricting using the standard exhaust leave the air inlet
snorkel and brass washer in the carb in place, and fit a 60 main jet.
Derestricting a UK Spec DNA 50
First remove the two nylon washers in-between the front pulley halves.
Remove the exhaust front pipe resonant tube (blind-ended tube) and weld
over the hole.
Rejet the carb with a larger main jet (standard is 56). Piaggio recommend
using a 58, but we prefer a 60.
Additional Performance
The next step is to fit an aftermarket exhaust system. We have found that
the LeoVinci Handmade ZX gives an excellent boost in performance when
fitted with the appropriate carb and transmission modifications (see Dynojet
graph 1 above).
The standard DNA carb has the usual "Hyper2" brass washer in the inlet
area. When this washer and the rubber air inlet snorkel into the air filter box
are removed, the carb needs rejetting with a larger main jet and pilot jet.
We used 36-38 pilot (standard is 32) and 74-76 main jet (standard is 56)
when we fitted a LeoVinci ZX exhaust system. With this setup power is up
to around 6.5bhp, compared to 3bhp when restricted and 5bhp when
derestricted. The transmission rollers need optimising (5 to 5.5g) and
stronger clutch springs need to be used (or an aftermarket clutch fitted).
If you are going to use a 70cc cylinder kit, the increased power at low RPM
means that the standard clutch does not need modifying.
To increase the performance further fit a 70cc cylinder kit, a gear kit and
transmission pulleys (Malossi front pulley and torque driver rear pulley
half). Because the standard carb is 17.5mm this is large enough for road
tuned engines.
Malossi make both cast-iron and aluminium MHR replica cylinders. The
latest Malossi cylinders have smaller spigots so that they can be fitted into
the smaller Hyper 2 crankcase area.The MHR cylinder is more powerful.
This is what we used to obtain graph 2 below.
DR cylinders have a larger bore at 48.0mm and the normal spigot size, so
if a DR cylinder is used the crankcase will need altering to remove the antitamper crankcase ‘bulges’. Other cylinder makes will need checking for
suitability. The latest Malossi front pulley and torque driver rear pulley
improve the transmission quality so that the engine RPM remains more
consistent throughout the speed range and therefore maximises the
acceleration.
This Gilera DNA 50 tuning guide should be read in conjuction with the
Hyper 2 tuning guide
Honda X8R, SFX and X8BR Tuning Guide
The Honda X8R, SFX and X8BR scooters are restricted by a rev limiter in the
CDI, a washer in the exhaust system and a restricted variator that limits gearing.
The washer in the exhaust front pipe is easily dealt with - just grind out the spot
welds and remove it.
The original CDI has two rev limiters built in, one at 7,500rpm and one at
9,250rpm. The standard motor runs at 7,000rpm at peak power.
The CDI can be modified to remove the 7,500 rev limiter. If the cylinder and
exhaust remain standard then this is okay, as by the time the motor hits the
9,250 rev limit power output is more than halved compared to power at 7,000.
The front pulley variator has restricted movement. The easiest way to fix this is to
replace the variator assembly. Either a cheaper replacement unit or, if further
tuning is envisaged, a Malossi or Polini unit.
The variator, CDI and exhaust modifications will give an increase in speed from
30 to 33mph up to 42 to 45mph. The power of the motor from the exhaust
modifications rise from 3.5 to 5bhp, giving enough power at 50cc to go to 50mph.
There is a modified Bali gear kit we do that is a 15% raise in gearing that gears
the 50 to the right level to achieve this.
If a 70cc kit is to be used the higher gearing from Malossi or Polini is more
sensible and increases the top speed potential to over 60mph on the standard
exhaust and 70mph on aftermarket exhausts.
If a higher revving exhaust or cylinder kit is to be used aftermarket CDI are
available that have programmable rev limiters to set to the RPM you wish or
bypass it completely.
With aftermarket exhausts (LeoVinci or Giannelli) that will give power above the
Honda 9250 rev limit. It is desirable to fit a CDI that will allow engine operation
above 10,000rpm to obtain full performance.
There are Malossi cast-iron and aluminium MHR 70cc cylinders available. The
road cast-iron cylinder with a suitable exhaust and carb kit will give over 8.5bhp
at the rear wheel. The MHR kit will give over 10bhp. Both will work with the
standard exhaust, but an aftermarket exhaust gives best results.
In order to achieve maximum increase in performance with the 70 kit - a larger
carburettor is necessary. Malossi do a 19mm carb.
In order to maximise acceleration the transmission can be improved by the use of
a torque driver rear pulley half, which minimises engine RPM changes.
When high gearing and higher revving cylinders and exhausts are used
acceleration from standstill can be improved by use of clutches that can generate
more slip. The Malossi fly clutch is the road version. The Delta clutch is the full
race unit that has adjustable springs and weight positions as well as a choice of
springs.
For most road engines stronger springs for the standard clutch or the fly clutch
are adequate.
For the ultimate road tune a full circle crankshaft with race spec conrod and big
end bearing is available. This allows modification to be made that allows
13,000rpm to be used reliably for race work.
To summarise:
Derestrict or change exhaust
Derestrict or change CDI
Change front variator
Fit raised gearing to suit 50 or 70cc
Fit 70cc kit, cast-iron or aluminium
Fit larger carb kit
Fit torque driver rear pulley to improve transmission
quality on high revving tuned motors
Fit race crankshaft
Disclaimer: We can not be held responsible for and damage caused by alterations to your
motorcycle/scooter this is intended for information use only.