MyScoot T-shirtHonda Ch125/150/250

      Most Honda scooters I've seen haven't worn out . They seem to be ridden for a short period and then parked. Sometimes a simple maintenance issue is neglected and the scooter is parked with the intention of fixing it when some "extra" money comes along. In any case there's a lot of older four stroke hondas available for good prices. In Canada where I live they are one of the few (aside from Yamaha) "modern " automatic scooters available over 50cc.  Quite a few people think they're ugly. Most everyone who has ridden them discover the real attraction and come away smiling . A couple of people from the local "classic scooter club" can attest to the fact that despite what you may think of their looks, Honda scooters are a lot of fun to ride. While I started out riding Lambrettas and still think they are better looking scooters , the Hondas are my "second choice" for riding. Even Piaggio, the makers of Vespa scooters, chose a Honda motor for one of their scooters.

    This page exists because there are a lot of used Hondas out there needing a bit of attention but otherwise perfectly capable of performing as low cost transportation and contributing to the general "mental health" of the population.  Hopefully this page will inspire their resurrection.

The Facts of Life

Some general facts taken from a 1986 European publication

CH 125(150)
CH 250
bore x stroke, mm
56.5 x 49.5 (58 x 57.8)
72 x 60
compression ratio / pressure in psi
10.3:1 / 156-212 psi
9.8:1 / 156-212 psi
piston clearance
0.010-0.040 mm
0.010-0.040 mm
ring gap 1/2/3, mm
0.15-0.30 / 0.15-0.30/0.2-0.9
0.15-0.30 / 0.15-0.30 / 0.2-0.7
Valve clearance (cold) intake
0.08-0.12 mm
0.08 - 0.12 mm (1989-90 is 0.10 - 0.14 mm)
"          "      "            exhaust
0.08-0.12 mm
0.08-0.12 mm (1989/90 is 0.10 - 0.14 mm)
intake valve timing open/close
0 deg TDC / 30 deg ABDC
5 deg BTDC / 30 deg ABDC
exhaust valve timing open/close
35 deg BBDC / 0 deg TDC
40 deg BBDC / 0 deg TDC
Carburetor type
keihin VE01 / VE01A (VE03A)
keihin VE05A 26.8 mm (1989-90 had 30 mm)
Main Jet
#98 (#100 on 150)
#115 (1989-90 models #112)
Pilot Jet
#35 (#35 on 150)
#38 (1989-90 had #40)
fuel/air where fitted
one and a quarter turns out
two turns out
Float height
18.5 mm
18.5 mm
Idle speed
1500 RPM
1500 RPM
Ignition timing@RPM
16 deg @ 1500 to 27 deg @ 3000
12 deg @ 1500 to 27 deg @ 6700
Spark Plug / gap
NGK DPR7EA-9 / 0.8-1.0 mm
0.032-0.039 in
NGK DPR6EA-9 / 0.8-0.9 mm
0.032-0.035 in.
Engine oil type/quantity
SAE 10w-30wSE/ 1000 cc
SAE 10W-30W SE/ 1000 cc
Bevel gearbox type/quantity
SAE 10w -30wSE/ 150 cc
SAE 10W-30W SE/ level plug
Drive Belt Length x Width 
 799 x 17 mm 
828 mm x 22.5 mm 
Coolant capacity
1110 cc
1350 cc
Fuel capacity
8.3 litres
8 litres (1989-90 had 9 litres)
12 volts / 9 amp hours
12 volts / 12 amp hours
12 volts / 45 watts, 45 watts
12 volts / 60 watts, 55 watts
front tire, size/pressure
3.50-10 4PR / 21 psi
4.00-10 4PR / 24 psi
rear tire, size/pressure
3.50-10 4PR / 28 psi
4.00-10 4PR / 28 psi
Cross Pollination
    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)

CH125 (11 ps)
XL125S (13 ps)
CB125RS (14ps)
    Bore x Stroke
56.5 x 49.5 mm
56.5 x 49.5 mm
56.5 x 49.5 mm
Intake Valve timing
   and duration
 open 00 deg BTDC
 open 30 deg ABDC
     210 deg 
open 10' BTDC
close 40' ABDC
   230 deg 
open 10' BTDC
close 40' ABDC
   230 deg 
Exhaust valve timing
   and duration
35 deg BBDC
00 deg ATDC
  215 deg 
open 40' BBDC
close 10, ATDC
   230 deg
open 40' BBDC
close 10, ATDC
   230 deg
   Ignition timing
16 deg @ 1500
   27 deg @ 3000 
10 deg @ 1950
   34 deg @ 3350 
10 deg @ 1400
 22 deg @ 3250
   compression ratio
Intake Valve dimensions(mm) 
Face diam x stem dia x length
26 x 4.98 x 81
31 x 5.5 x 88.3
Exhaust valve dimensions(mm)
Face diam x stem dia x length
 22 x 4.97 x 81
26 x 5.5 x 88.8

        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 (  Honda TL 125 FAQ page ) 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 offers 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.

Honda four stroke scooter exhaust 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  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.

Honda scooter cylinder head cross-sectionWhile 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. Holding a ch125/150 camshaft next to a ch/cn250 camshaft

 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. 


     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.


    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 "apple" which seems to work far better and produce more power than seemingly "identical" motors. At one point, manufacturers used to keep the apples 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".

stock inlet port- CH150
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.

Elite 150 intake manifold matched to ported and polished head 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. 

    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.

Exhaust manifold drawing

  Honda four stroke scooter
  exhaust manifold where it bolts
  to the motor. 

If you aren't in the mood to pull the head off the motor and disssemble 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 cople 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 diameter 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. 

    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.

"Bolt On" Performance

    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 while your scooter is being experimented upon. 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.

    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. BeedSpeed scooter shop has an online scooter catalogue which should get you started and eagerly awaiting the day when  those stock parts need replacing. If you can read German, download Scooter Center Kolns automatic scooter catalogue. Even if you can't read German,  you'll be pleasantly surprised by the range of parts and the pictures help you figure out what  parts they are offering.

Reflex variable pulley weightsBeing "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 15 year old Helixes. Scooter Center Koln has a catalogue that can be downloaded which contains a variety of aftermarket weights to experiment with.  The more expensive alternative is to buy an aftermarket variable pulley from BeedSpeed that helps the Reflex accelerate as quick as it looks. Check out your local scooter shop that sells Malossi, Polini etc weights .

4 stroke scooter motor
Serious Overclocking
       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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Resource sites:
Honda Scooterist  Bitubo Shock absorbers BeedSpeed Scooter parts
A Helix Page Scooter Pros and Cons BikeLinks ~ a fountain of info
ScootDawgs Chinese
Honda clone page

Sudco (American distributor of mikuni and keihin)
Helm Publications (workshop and owner manuals)
Malossi UK  250 Elite Road Test Vincent Crabtrees Honda SOHC site
Honda Scooters So you bought a used Scooter Carburetor rebuilding
How stuff works Honda SOHC engine modification Honda scooter seatcovers
CV carburetor  Even more parts (England) Scooter Therapys Tire changing tips
Hagon Shock absorbers 1984 review of a ch125"...the most advanced scooter in the solar system.."
LeoVinci Exhaust systems
Riders Tips Honda SOHC owners club Honda Scooter Revolution
Exhaust design software First Moto Honda Models Honda Reflex Club

Links to my other scooter pages

Japanese Scooter index Used scooter
scooter ID
Elite 80 Used 
Scooter parts 
Honda scooter owners Index
Elite 50cc 
Aero 80cc
125/ 150/ 250 
50 cc Honda