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The Facts of Life
Some general facts taken from a 1986
European publication
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CH 125(150)
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CH 250
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bore x stroke, mm
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56.5 x 49.5 (58 x
57.8)
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72 x 60
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compression ratio /
pressure in psi
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10.3:1 / 156-212
psi
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9.8:1 / 156-212 psi
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piston clearance
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0.010-0.040 mm
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0.010-0.040 mm
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ring gap 1/2/3, mm
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0.15-0.30 /
0.15-0.30/0.2-0.9
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0.15-0.30 /
0.15-0.30 / 0.2-0.7
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Valve clearance
(cold) intake
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0.08-0.12 mm
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0.08 - 0.12 mm
(1989-90 is 0.10 - 0.14 mm)
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"
"
"
exhaust
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0.08-0.12 mm
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0.08-0.12 mm
(1989/90 is 0.10 - 0.14 mm)
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intake valve timing
open/close
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0 deg TDC / 30 deg
ABDC
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5 deg BTDC / 30 deg
ABDC
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exhaust valve
timing open/close
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35 deg BBDC / 0 deg
TDC
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40 deg BBDC / 0 deg
TDC
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Carburetor type
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keihin VE01 / VE01A
(VE03A)
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keihin VE05A 26.8
mm (1989-90 had 30 mm)
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Main Jet
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#98 (#100 on 150)
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#115 (1989-90
models #112)
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Pilot Jet
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#35 (#35 on 150)
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#38 (1989-90 had
#40)
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fuel/air where
fitted
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one and a quarter
turns out
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two turns out
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Float height
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18.5 mm
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18.5 mm
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Idle speed
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1500 RPM
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1500 RPM
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Ignition timing@RPM
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16 deg @ 1500 to 27
deg @ 3000
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12 deg @ 1500 to 27
deg @ 6700
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Spark Plug / gap
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NGK DPR7EA-9 /
0.8-1.0 mm
0.032-0.039 in
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NGK DPR6EA-9 /
0.8-0.9 mm
0.032-0.035 in.
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Engine oil
type/quantity
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SAE 10w-30wSE/ 1000
cc
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SAE 10W-30W SE/
1000 cc
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Bevel gearbox
type/quantity
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SAE 10w -30wSE/ 150
cc
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SAE 10W-30W SE/
level plug
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Drive Belt Length x
Width
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799 x 17
mm
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828 mm x 22.5
mm
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Coolant capacity
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1110 cc
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1350 cc
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Fuel capacity
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8.3 litres
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8 litres (1989-90
had 9 litres)
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Battery
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12 volts / 9 amp
hours
YTX9-BS
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12 volts / 12 amp
hours
YTX12-BS
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Headlamp
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12 volts / 45
watts, 45 watts
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12 volts / 60
watts, 55 watts
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front tire,
size/pressure
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3.50-10 4PR / 21
psi
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4.00-10 4PR / 24
psi
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rear tire,
size/pressure
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3.50-10 4PR / 28
psi
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4.00-10 4PR / 28
psi
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Cross Pollination
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From the same 1986 Publication as shown
above I've obtained some figures for Honda engines similar , if not identical
in some respects, to the CH125. These are the XL125S, CG125, CB125RS; all air
cooled single cylinder SOHC motors with the same bore and stroke as the CH125
from the same period. The various specifications are given to show the tuning
possibilities of the 125 scooter motor ~all the other motors are in a higher
state of tune. Vincent
Crabtree has a good site detailing some of the characteristics of Hondas
SOHC two valve singles.
1 "PS" equals about 0.986 horsepower. The following
should give some idea of relative state of tune. The 1983 Spacy
produced 11ps @7,500 RPM whereas a 1975 XL125 produced 13ps @ 9,500 RPM, a 1975
cb 125RS producing 14 ps @ 10,000 RPM. So why exactly do all these
engines of equal displacement have such different characteristics? The
following gives a glimpse of some of the differences.
Honda SOHC motor
comparison (1986 Specs)
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CH125 (11 ps)
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XL125S (13 ps)
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CB125RS (14ps)
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Bore x Stroke
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56.5 x 49.5 mm
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56.5 x 49.5 mm
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56.5 x 49.5 mm
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Intake Valve timing
and duration
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open 00 deg
BTDC
open 30 deg ABDC
210 deg
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open 10' BTDC
close 40' ABDC
230 deg
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open 10' BTDC
close 40' ABDC
230 deg
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Exhaust valve
timing
and duration
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open 35 deg BBDC
close 00 deg ATDC
215 deg
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open 40' BBDC
close 10, ATDC
230 deg
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open 40' BBDC
close 10, ATDC
230 deg
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Ignition timing
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16 deg @ 1500
27 deg @ 3000
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10 deg @ 1950
34 deg @ 3350
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10 deg @ 1400
22 deg @ 3250
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compression ratio
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10.3
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9.4
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9.3
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Intake Valve
dimensions(mm)
Face diam x stem dia x length
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26 x 4.98 x 81
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?
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31 x 5.5 x 88.3
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Exhaust valve
dimensions(mm)
Face diam x stem dia x length
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22 x 4.97 x
81
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26 x 5.5 x 88.8
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There are several companies out
there offering performance enhancements for the Honda 125 motor and some should
be applicable to the scooter motor. I found a web site
with tuning information on the Honda 125 SOHC motor. While the
water-cooled "CH" engines are not identical to the air cooled engines
mentioned, they seem similar enough that some of the tuning tips for the
TL/XL/CB range should apply to the scooter motor. The most likely
adaptations seem to be porting and polishing, and cam regrinding. The fitting
of Carburetors and exhausts to improve the breathing ability would also be an
asset. As an example of a typical "upgrade" Malossi
offered a 28 mm carburetor kit for the XL125. Compare that to the 22mm effective
bore(26mm venturi) of the CV carburetor stock on 125/150 scooters.
Local motorcycle tuners who have experience modifying parts for your particular
engine type (Honda SOHC single cylinder such as XL/XR Honda dirtbikes) may even
be able to offer some advice as to what type of modifications would best suit
your style of riding. Ask around at the local motorcycle shop. Honda factory
dealers may or may not be a help.
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LEFT: rough sketch of Elite/Spacy exhaust
Most motorcycle shops could either fabricate a new exhaust system or
recommend a local welder who could. Be prepared to pay a bit of money as the
scooter exhaust is more complicated than a motorcycle but the bare materials
such as tubing and a generic muffler should not be any trouble to obtain. The
length of exhaust and size of tubing affects a motors performance. Generally,
a shorter exhaust length would perform better at higher rpm and vice versa.
The scooter exhaust still needs to be longer than the space between the
exhaust port and the back of the scooter. The ideal exhaust for the Honda scooter motors ends up
looking like a paperclip where it has to do one loop before exiting to the
rear. Check out this website for
some software that will help you figure out the ideal exhaust length for your
scoot.
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 Andrew
Fitted a ch150 motor to his Honda Cub commuter after two stock
engines blew up on his highway commute. The exhaust was
fabricated
from an old VW beetle manifold. After fitting the new exhaust, he
re-fitted the stock exhaust to do a comparison and he had this to say
of the results:
I decided as an experiment, to repair and re-install the stock
exhaust muffler on the engine. The engine
performed well up to about 40 mph. I noticed the engine working
harder in the 40 to 50 mph range, and did not run up to 60 mph anywhere
near as enthusiastically as before, and it seemed to require more twist
on the throttle to achieve the same speeds and acceleration
as before. I wanted to examine my homemade header to see if I could
make it a little more civilized. I dismantled my hastily assembled
glasspack muffler, and built a new core of finely perforated steel
sheet. The holes are 5/64ths of an inch, and are closely spaced. The rolled perforated piece is 1 in. dia.
and 10
in. long. I machined a ring for each end to support the baffle in the 1
1/2 in outer tube. I wound fibreglass roving around the inner
piece (enough to require a light push) to get it together,
installed the outer retaining ring, and swapped exhaust systems again.
Road test. Startup sounded real good, not a wimpy putter like the stock
pipe, but a nice purr. Acceleration was indistingishable from the loud
pipe, and the engine was again happy to explore its higher rpm
powerband.
Read more about Andrews conversion at this page. |
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While I've read that the 250cc engine has room for boring the
cylinder, the 150 Elite cylinder doesn't have a lot of meat to remove.
The steel cylinder liner is only a little under 3 mm leaving little room
for anything much larger than factory oversize pistons. The Elite 150
cylinder head as well doesn't leave a lot of room for larger valves. Even
compared to the other 125 cc SOHC Hondas it seems to be undervalved (see chart
above). There's not much space between the valve seats on the CH150 for
installing larger valves, especially the 31 and 26 mm valves from the CB125. I
looked at some similar size Hondas to see if a larger valve could be easily
swapped but the scooter valves are quite different in the length, stem diameter
than other Hondas and the swaps that I could see would require doing some major
modifications to the valve, valve guide and head etc. to make them fit.
An interesting fact to note is that the cylinder
mounting bolt pattern for the CH125/150 and the CH/CN250 are identical. While
it wouldn't be a straight "bolt on" conversion it should be possible
to mount a modified 250 head onto the smaller motor and achieve a very large
gain in valve size and breathing. The larger valves, ports and the larger
30mm carburetor attached to the head would do a lot to increase the power.
At this time I've decided to just polish the valves and ports to help
the breathing. I think the main modifications will consist of a different
carburetor (I have a spare 26 mm VM mikuni hanging about), less restrictive
exhaust and a re-profiled camshaft to allow timing more in line with the other
125 Hondas.
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I've heard from CH125
owners who've fitted the 150 piston. It won't give you 150cc but
is apparently an easy swap otherwise. The 125 piston is domed (higher
compresssion) while the 150 piston is flat topped and an interesting
conversion would be to fit the 125 piston and cylinder to the 150 motor. Its
not a "bolt on" item as the piston would have to be modified for
crankshaft and valve clearance.
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The stock Honda scooter motor could take you around the world with its inherent
reliability and low state of tune. I do think that it is possible to
raise the state of tune and still retain the reliability.
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BluePrinting |
When brand new motors are produced, they're not all
identical. We've all heard of the term "lemon" referring to a vehicle
that just doesn't seem to work even though it has "all new" parts and
identical models work just fine. At the other end of the scale from the lemon
is the "peach" which seems to work far better and produce more power
than seemingly "identical" motors. At one point, manufacturers used
to keep the peaches aside and use them in factory racers because they produced
"extra" power. When a race requires "stock" motors then
tuners usually tear apart a motor and rebuild it to designed
tolerances. If you are rebuilding or even just doing a top end
overhaul, then you should consider doing a "blueprinting" job on the
cylinder head. The reason for the difference in power between engines is
because all motor parts have a certain tolerance factor where they can be
minutely different from the original design or "blueprint".
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LEFT: CH150
stock intake port
While the original design drawings undoubtedly show a smooth opening
from the carburetor mouth to the point where the gas mixture enters the
combustion chamber, the reality is that casting marks, small differences in
mating surfaces and imperfections in the metal detract from the smooth flow
of air through the intake tract.
An Elite 150 motor has a 26 mm carburetor mouth connecting to a 23 mm
manifold which was in turn attached to the cylinder head (where the opening
did not match up correctly) opening of 22 mm (at its widest). The
scooter motors exhaust and intake passages are not overlarge to begin with so
any small amount removed will enhance air flow and improve the amount
of air/fuel mixture. Just don't forget those water passages when you're
removing material from the ports.
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The picture at the left is an Elite 150 intake after
matching up the openings and porting . The manifold to cylinder port mating
is at the dark line shown and is no longer mismatched as on the
original. A dremel tool with a drum sanding head was used to open up the
passageways and achieve an almost seamless opening from carburetor to valve
opening. Several finer grades of sandpaper, a stainless steel
wire wheel and polishing compound have since been used to polish the ports .
The desired finish for the intake port is more of a swirled effect than a
"mirror finish". Exhaust ports can be as shiny as you can get
them.
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Ensuring that all intake openings match up and all casting
marks are removed will make a difference in the amount of fuel getting
into the combustion chamber and increase power. Remember that there
are water passages behind those intake and exhaust port walls so smoothing the
irregularities on the surface and between mating surfaces is the key rather
than seeing how much metal can be removed. Unlike other "performance"
improvements this won't be over stressing the engine; simply ensuring it
conforms to the original design. Don't be tempted to grind away too much. That
could actually hurt bottom end performance by reducing the velocity of the
gases as they pass through the ports. Motors designed for bottom end torque
sometimes have their intake ports made smaller to increase the velocity of the
intake gases. A smooth mirror like finish is good for the exhaust port but for
the intake , a satin finish works best. A satin finish helps to keep the fuel
atomized and in a gaseous state instead of liquefying on the walls of a port
with a mirror like finish. |
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Honda four stroke scooter
exhaust manifold where it bolts to the motor.
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If you aren't in the mood to pull the head off the motor
and disassemble the top end to polish the ports, an improvement can
be made by just matching the ports with their respective manifolds . In
the case of a 250 cc motor I'm working on, I don't want to take it apart due
to cost and time restraints as I'm fitting it to a Helix with a blown motor
I want to use for daily transport. The motor runs just fine but I have it out
of the scooter at the moment so thought I'd just do a bit of matching. In
smaller 50 cc scooters they use washers welded into the manifolds to reduce
the diameter and thus power so the scooters meet the local "moped"
laws. This same principle on the larger scooters ( 30 mm manifold matched to
a 27.5 mm port ) has a similiar effect of "restricting" the
power.
On the 250 motor, the exhaust is held on by a casting which is in turn
welded to the tube running to the muffler (drawing at left). When the
casting is welded to the tube, the weld protrudes a couple of mm into
the exhaust path. On my particular scooter the 26.3 mm exhaust port
attached to the 25 mm tube but the weld constricted the tube to less
than 23 mm. A substantial increase in area (18%) could be had by simply
filing away the weld bead. A further increase could be had by bevelling
the casting opening out to 27 mm so the exhaust gases don't hit an
abrupt edge as they are trying to exit the motor. Smooth transitions
from one size to the next aren't as harmful to flow as an abrupt
change.
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One of the advantages of having a factory service manual is that they have
most of the factory tolerances listed. While some can be measured by
inexpensive feeler gauges, others may need some fairly sophisticated measuring
tools. Odds are good that a blueprinting job can be done locally as,
aside from knowing the tolerances for your particular motor, most of the
blueprinting skills can be applied to any motor although for the four stroke
motors, I'd stick to a four stroke specialist. While it deals with two
stroke engines the MacDizzy site
has some good pictures of cylinder casting irregularities and ways to go about
smoothing out the intake tract. Removing large amounts of metal is not
usually required . Smoothing and polishing is the way to go.
A new set of roller weights with a kevlar belt,
high flow air filter, free flow exhaust along with rejetting of the carb should
offer a noticeable difference in power (and the weight of your wallet).
Oh yeah - don't forget a decent set of tires. Scooter Therapy
has an excellent page on how to change tires. All the four stroke
Elite/Spacy/Freeway have tubeless rims. Bridgestones are a
favourite of mine as they offer good performance at a reasonable cost. If
I lived in Europe I might go for the Continentals or Michelins.
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"Bolt On" Performance
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| Many of the improvements to performance already mentioned
involve quite a bit of commitment on the part of the owner (time, knowledge and
expense). There is also a degree of risk in that there is little published
information on tuning four stroke Honda scooters and a lot of pioneering spirit
is required as well as down time .
Most owners of Hondas are looking only for modest improvements that they
can accomplish in a day in their own workshop. Improvements are most likely
only considered when a part has to be replaced as a part of the scooters
regular maintenance . Likely parts for replacement are drive belts,
exhaust mufflers, tires, shock absorbers, variators or just the variator
weights. All the above mentioned will likely need replacement well before the
scooter is in need of a major overhaul.
The most overlooked source of improving performance
is a simple tune-up. Replace all parts at or before the recommended
intervals in the owners guide and if you drive anything like me, well before
the factory intervals. Normal wear in the stock parts (belt, roller weights,
engine oil, valve train, spark plug, air filter etc) can substantially
reduce acceleration and top speed even when the motor appears to be running
like a clock. Clogged
up main jets and idle jets are probably the most likely culprits when it
comes to carburetor "problems". Just soaking the carburetor in
cleaner won't solve this. Fuel "problems" seem to happen often with scooters that are "stored" for lengthy periods.
The second most overlooked performance improvement is
tires and aftermarket shocks. Switch the Cheng Shins for a set of Bridgestones/Michelins
etc. . Research the type of tire suited for your climate. What's best in rainy
Britain is not what's best for dry California. The tread type, the tire
profile, the rubber composition and even the size are all critical factors in
determining what tire is best suited for you. Tires suited to your
scooter, driving style and climate will make an incredible difference.
You'll especially notice a difference in cornering, on braking and on rainy
days. If you live in a desert with highways devoid of
traffic/animals/bumps/potholes that run straight until the horizon then you
don't need good tires but if you live in the real world I'd put tires near the
top on the performance list. Consider the fact that manufacturers ship the same
tire on your scooter whether you live in a rainforest or a desert.
Aftermarket suppliers like Malossi, Bitubo, LeoVinci,
Giannelli and Daytona as well as the various tire manufacturers offer parts
that will increase power, increase engine flexibility, offer improved braking,
suspension damping and handling to create a much more pleasurable ride.
And hey~ you were going to spend the money on that part anyway so the little
extra you spend for that performance part isn't such a stretch. With the cost
of stock Honda parts you may even end up saving money and increasing
performance at the same time.
 Being
"economically challenged" I could never afford a brand new scooter
however on the introduction of Hondas Reflex scooter I managed to procure a
brand new factory service manual. I found several postings on alt.scooter about
the Reflex being slower than a Helix in acceleration which was surprising.
Owners also mentioned a "two-speed" transmission . A review of the
manual shows a drive similiar to all other auto scooters except that the Reflex
uses two different roller weights in the variable pulley mechanism. For owners
of the Reflex that want increased acceleration I would think an easy "fix"
would be to replace the two different roller weights with just one
weight. The 3 heavier beige roller weights could be replaced with the
lighter black roller weights which should yield quicker acceleration allowing
you to keep up with those 20 year old Helixes.
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Serious Overclocking
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As a final
note on the possibilities of tuning I came across a posting from
"Frank" of some modifications he did to his Elite 250 motor. Frank
used some basic four stroke modification practices and applied them to his
Honda with some interesting results. Except for the Malossi parts, Frank used
local companies and his own skills to fashion the parts necessary to transform
his once mild mannered 1989 ch250 Honda. This is by no means "bolt
on" performance and would require competent mechanical skills as
well as patience while getting the motor set up. If increases of 10 to 15 mph
in top speed and wheelies sound like something you'd be interested in
then you'll want to read the High performance Honda Page
which also includes details of an Italian performance modification
for 250cc scooters.
While both of the aforementioned modifications involve
250cc scooters, another scooterist has applied some similiar techniques to his
125cc Honda clone which could be applied to any four stroke scooters. Read this page for some
further tips on tuning the 125/150/250 and why you shouldn't stop just because
someone says they don't make performance parts for that scooter.
One of the most economical and straightforward methods
of increasing power is to increase engine capacity and I've received an e-mail
from a reader who has come up with an ingenious way of doubling the size of his
CH125. Click here
for further details. |
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