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What follows is a basic overview of the carburetor used on Honda four stroke scooters and is meant to aid an owner in a very basic understanding of how this particular fuel-air meter works. If you've been quoted $200 to have someone "look at the carb" this page will hopefully give you enough information to decide whether its something you could reasonably do yourself and spend that money on some more worthwhile cause. Even if you do have someone else perform the work, you'll have a better idea of what is involved. In my opinion the most difficult task is in removing the carburetor after which working on it would be a piece of cake. While it only involves loosening a couple of bolts and screws, the amount of tubing and wires Honda likes to connect to its carburetors is astounding and the first issue you'll have to deal with is how to remember what goes where. Some helpful hints and links are included in the text to make that task easier.
A "constant velocity" carburetor (CV carb) is used on Honda Elite CH125, 150 and 250 and on the smaller Elite CH80 as well as the 50cc Ruckus and Metropolitan/Jazz scooters. On conventional carburetors the throttle cable is connected directly to the throttle slide. When you twist the throttle, this lifts the slide and immediately increases the size of the carburetor opening letting in more air/fuel mix and increasing the speed of the motor. On CV carburetors, the throttle cable actuates a butterfly valve (just a lever on the CH80) and, as the throttle is opened, the air pressure difference between the sealed chamber above the vacuum slide and inside the carburetor venturi forces the slide (located in front of the butterfly valve) up and down. The downside to the CV carb is a lack of immediate throttle response. Twisting the throttle gives relatively leisurely acceleration compared to a conventional carburetor. One of the advantages is that the carburetor adapts nicely to altitude changes and good gas mileage .
Experiencing a problem with the carburetor on your scooter can be frustrating if you're unaware of the different parts and their function. A visual inspection of the carburetor should be done regularly and especially when problems arise. While checking the carburetor look for:
- fuel leaks,
- cracked, kinked, leaking, clogged or disconnected hoses
- clogged fuel filter
- loose or disconnected airfilter housings~ this is quite common due to the movement of the engine and will have quite an effect on how smooth your engine runs.
- make sure the throttle works smoothly and opens to the proper full throttle position. In order to check that the throttle is opening you'll have to stick your finger in the carb mouth and push up on the vacuum slide. There should be some resistance to pushing the slide up but it should move smoothly once some pressure is put on it and then lower itself once the pressure has been removed.
The following comments assume you've removed the airfilter housings, have disconnected all the hoses, cables and electrical connections (and identified them for replacement) and have the carburetor drained of gas and removed from the scooter. I'd recommend drawing a diagram of the carburetor before disconnecting the various hoses. Attaching masking tape with written descriptions is also an option but the writing could become smudged once you start cleaning the carburetor. DO NOT... DO NOT rely on your memory.
For scooters that have been sitting long periods of time then a carburetor rebuild is probably in order. The carburetor is a fairly simple mechanism and the parts that need to be inspected are fairly easy to get at once the carburetor has been removed from the scooter. A Honda service manual would be nice but, in any event, check out Old Man Hondas web page for a good overview of rebuilding and/or cleaning a carburetor.
A clean,
well lit work area and something to organize the tiny bits (egg carton
etc.) is necessary. I like to clean the outside of the carburetor first
before disassembling it ~ usually while it is still on the bike. By not
cleaning, you could actually allow grit and dirt to enter the fuel system
when removing the carb doing more damage than if you'd simply left it alone.
Any number of carburetor cleaners (Gunk etc.) are available from the local
auto parts store. Many of the parts are very small and hard to handle so
when you're disassembling, have a large white area (old pillow cover, towel,
etc.) underneath so any parts that accidentally fall out will not bounce
away into oblivion like those socks in your dryer. Old
Man Hondas web page has a good pictorial display of the different parts
you'll find and how to determine if they are in need of replacement. The
following items should be carefully inspected after you've washed the parts,
your hands and moved to a clean, well lit work
area:
Keihin CV carburetor from a 1989 CH250 |
Main jet and Main jet holder~ #19 and 20 in the drawing
at top. After removing the main jet with a flat blade screwdriver, also
remove the jet holder with a 7 mm socket/wrench. Brand new they looked
like bright shiny pennies but yours will undoubtedly look quite tarnished.
That tarnish on the outside isn't as much a problem as the same tarnish
on the inside of the various openings which has decreased their size. Soaking
in Gunk and cleaning by compressed air MAY help remove some of the
tarnish and restore the openings to their original size. Replacement may
be needed if cleaning doesn't help. A new main jet is one of the cheapest
"performance " parts you will ever buy.
Slow or Idle jet
~
#22 in the drawing at top. A very small screwdriver head is needed to remove
the idle jet which sits right beside the main jet but inside a tube. The
size of this jets opening is considerably smaller than the main jet and,
where tarnish may have reduced the main jets opening by 10% it may very
well have almost sealed the idle jet completely closed. If your idle has
been deteriorating over time, this is probably the culprit. If you were
going to replace only one piece this is most likely the one that needs
replacing. On the other hand, the "B" string from my guitar (0.356 mm)
is just about the right size to clean out any accumulated grunge from the
idle jet (#35) on my Elite 150. Guitar strings can usually be bought individually
at your local music shop. The drawing of the CV carb above shows an air/fuel
adjuster ( #23 ) but my 26 mm carb has no such adjuster. One less thing
you have to worry about going out of adjustment.
Vacuum slide and
diaphragm ~# 8 in the drawing
at top. This is actually just a one piece item and is accessed by first
removing the two screws at the top of the carburetor. Note the position
of the parts for re-assembly before removing the spring, the nylon keeper
and the needle. . The diaphragm has a round extrusion on the edge that
helps position it correctly. The rubber should not show any cracks or signs
of wear. Neither should the slide show any resistance to moving up and
down in its opening. The only resistance should have been from the spring
and vacuum after the piece has been re-installed. If the engine idles but
will not increase speed with throttle, a damaged diaphragm is likely
(or your throttle cable is broken). Check the diaphragm for damage by stretching
the rubber and holding it up to a light . If there are holes, the
rubber looks worn or the slide is damaged ,the entire slide/diaphragm assembly
must be replaced. Also check that the hole next to where the needle sits
is not blocked. This hole is what allows a vaccum to build up in the chamber
above the slide and cause the slide to raise once air velocity in the carburetor
mouth increases. Some websites I've seen have reccomended opening this
hole slightly which would, I imagine, decrease the response time between
opening the butterfly throttle and the slide opening. I haven't been able
to find any other information on this subject but it re-inforces the importance
of ensuring the "vaccum port" is not obstructed. Loss of power and/or low-speed
lurching when the engine is hot, and erratic performance can sometimes
be
traced to a sticking vacuum piston in the carburetors. It's easy to cure
- just remove the carburetor caps, clean the chrome piston with solvent,
replace and reassemble. (Do NOT lubricate the piston.)
Float
Valve
~ #18 in the diagram above.
If your carburetor has higher than expected gas consumption and is dripping
gas, this could be the culprit. After you've removed the float, you'll
see the float valve hanging from it. When the float bowl fills up with
gas, the float pushes on the float valve which in turn shuts off the flow
of gas to the carburetor until the gas level goes down in the float bowl.
The float valve in the Elites generally have rubber tips so they last longer
but if you can see the slightest bit of wear in the pointy part (usually
a ring around) then replace it.
Jets don't cost a lot of money and replacing them is a surefire method of restoring the correct size jet opening. If the Jet doesn't look covered or plugged with varnish a better method is to simply soak the jet in carburetor cleaner and blow it out with air. That goes with all the internal passages in the carburetor once the jets etc. have been removed. A good point to re-inforce here is that your work area should be very clean as something the size of one grain of sugar could wreak havoc inside the carburetor. I don't have a compressor so one method I've used is the bicycle pump and basketball inflator method. You'll need one of those long pin shaped thingies that fits on the end of a bicycle pump that is used for filling up basketballs/footballs/soccer balls etc. The end that fits into the ball usually has two holes mounted in the side of the pin. I cut off the end of the pin and smooth it out so the air will be directed straight out the end of the pin into the carb/jet I want to clean. Cheap but effective. The local gas station usually has a compressed air hose for filling tires with much more pressure if needed but the pin thingie will still come in handy. The more air pressure, the greater the possibility of creating a deadly projectile so use extreme caution.
Before re-assembling,
ensure you've checked all gaskets and, if you're like me, you'll simply
replace all gaskets because this is one thing thats "relatively" cheap
to replace. In fact, a damaged gasket could be the reason for the carburetor
acting up and don't forget the rubber O-ring that fits between the manifold
and the cylinder head. A bit of oil rubbed on the inner opening of the
rubber intake manifold will also ensure that the carburetor can be re-mounted
easily. There is a notch at the top of the intake manifold that fits into
a corresponding "tab" on the top of the carburetor on my Elite 150. Check
your scooter to see if any such arrangement exists. Re-connect all hoses
after a quick inspection to make sure they aren't cracked or broken. Re-connect
the throttle cable and wiring to the automatic choke.
Most scooters have an "electric choke" which acts automatically to help the scooter start. There are two parts to this system. The first is the set of passageways internal to the carburetor ( fuel enrichment circuit). The second is the big black lump sticking out of the carburetor (photo at left) with two wires that connect to the rest of the wiring loom(bystarter valve).
The Fuel enrichment circuit acts like the manual choke in other carburetors to aid cold starts . Instead of reducing the air intake to richen the fuel/air mixture like a "choke", the "fuel enrichment" circuit adds fuel to the existing mixture. The "fuel enrichment circuit" has a separate jet that picks up fuel from the fuel bowl and sends it to the venturi of the carburetor. The function of the Bystarter valve is to close off this circuit after the engine has warmed up and no longer needs an "enrichened " fuel mixture. The brass needle shown sticking out of the bottom of the bystarter extends to block the "fuel enrichment" circuit once the scooter is started and electricity is generated to the bystarter valve. It takes time to fully extend, thus giving the scooter time to "warm up".
I get a lot of mail asking about where to get a new bystarter valve. The writer usually goes on to explain that their scooter is hard to start when cold so the bystarter valve must be "broken". The flaw in this reasoning is that if the bystarter valve is "broken" and not extending, then the scooter would be easy to start when cold but hard to start when warm. Odds are if your scooter is hard to start, it's the fuel passage that's clogged (idle, choke or both; see photo below) .
Both jets are very small and prone to clogging if you let the scooter sit for long periods of time without running. If you do not plan on running the scooter for lengthy periods then drain the fuel bowl. Unlike the idle jet, the fuel enrichment jet isn't replaceable. It's simply a small hole drilled into a brass rod that's then pressed into the carburetor body . The idle air/fuel adjustment screw shown in the photo isn't present in all carburetors. The carburetor shown isn't off of a Honda scooter but was simply a carburetor I had available for taking a pic of. If you take the fuel bowl off of the Honda carburetors it will look very similiar, if not identical, to the one in the photo.
Replacement
carburetors
At
some point you may consider replacing the stock carburetor. Even
though the carburetor is potentially just a "bolt on" addition, you'll
need to ensure the jetting is set up correctly before you can realize the
new carbs full potential. It will take some time and a local motorcycle
shop may be able to help. By just bolting on a bigger carburetor, you could
actually see a performance decrease. As mentioned on the Elite page, you
should fit any performance pieces at once (air filter, exhaust, cam, carburetor)
and then tune the scooter with everything in place. One thing you'll have
to give up is the automatic choke. Manual chokes controlled by cable are
my prefernce in any case.
The CH125/150 and CH250 have a a pretty tight space and so you should
also ensure there's enough room for any aftermarket carburetor taking
into account the engines movement.
Although the Keihin
CV carburetor on my Elite 150 has a 26 mm bore at the manifold end,
the air filter end has an oval bore equivalent to a little over 22 mm diameter
. Fitting a 24 mm or larger throttle slide carburetor would offer an improvement
in the amount of fuel availiable, better throttle response and aid in higher
engine speed. The size of the inlet manifold would not have to be modified
much to accomodate a 24 to 26 mm slide carburetor although retailers such
as Pro-Flo offer
manifolds for replacement carburetors such as the VM26 Mikuni that
could be adapted to the Elite. My Elite 150 has 57 mm bolt hole centers
on the intake manifold. As an example of a typical upgrade, Malossi
offers a 28 mm carburetor kit for the Honda XL125 which has a similiar
configuration to the Elite 125/150 motor.
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An interesting web page on tuning dellorto carburetors provided the graph shown at left. It shows what size carburetor is appropriate for a certain horsepower. It is a very general guide but is useful in illustrating the range of carburetion suitable for a specific engine. Just going by stock horsepower figures, the range of carburetors for a 250 cc Honda scooter motor (19 hp) would range from 26 mm to 32 mm. The range for a 150 cc Honda scooter motor (11 hp) would be 22 mm to a bit over 26 mm. The stock carburetion on both scooters falls in the bottom of each range and if a hotter cam, higher compression and sport exhaust is fitted, the stock scooter quickly becomes inadequate . The CV series carburetors equivalent size is smaller than the venturi area . A 30 mm CV carb probably has the same venturi size as a 26 mm round slide mikuni. |
Several different types of carburetors are available for use on scooters. The most common are the slide types and the constant velocity type. While the constant velocity types have slides, the slides are not directly controlled by the throttle cable. Butterfly type carburetors are also available but are generally not as easy to obtain. Some carburetors also have accelerator pumps which will get rid of the annoying "stumble" when the throttle is first opened. An accelerator pump or the equivalent (power jet etc.) will pump out extra gas when the throttle is opened to aid ... you guessed it, acceleration.
Constant velocity carburetors are common on street bikes and produce smooth acceleration and reduce the engine "bogging" when accelerating . With a CV carburetor the speed at which its slide opens is dependent more on the presence of a vacuum than how fast you open the throttle. Crack the throttle wide open quickly and the slide will open only as fast as vacuum builds. If you're a rider who enjoys the leisurely acceleration of the engine to 6,000 RPM, the stock carb is just fine but even Honda increased the size of the carburetor on the 250 over the years .
If you're a rider who likes quick acceleration to high RPM) , the stock carburetor has its limitations and you may consider changing the carburetor. You could simply move up to a larger venturi size of CV carburetor however you would still be left with the leisurely response to throttle openings. Having a Motorcycle wrecking yard handy might provide a low cost carburetor for experimentation without having to invest a large sum of money. More than a few motorcycles/scooters/ATVs came with CV carbs fitted and finding a larger model at a wreckers shouldn't be difficult.
Slide carbs will perform better at higher RPM's and generally have a quicker response time than a similiar size constant velocity carb. Even slide carburetors come in a variety of models with flat slides, round slides, oval venturis etc. thrown into the mix just to make the whole issue even more confusing. Do make sure that the carburetor you want is designed for a four stroke engine as differences in throttle slide design exist between two and four stroke carburetors. My preference would be a "VM" series mikuni carburetor as they are inexpensive and suited to four stroke motors.
Changing carburetors
is rarely just a "bolt on" operation with jetting, choke operation and
mounting being just some of the issues involved. For North Americans,
Sudcos
website shows a variety of Mikuni and Keihin carburetors in several designs,
size and method of operation. They also offer a service (special set) where
the customer describes the bike they are fixing the carb to and Sudco provides
the correct jetting for the intended application. The following is
an excerpt from Sudcos web page and explains the information they need
to set up your carburetor:
- Sudco can "Special
Set" any Mikuni Carburetor for your particular application. Below is the
information you will need to provide our Carburetor Techs so they can provide
you with the correct carburetor and jetting, when you are purchasing a
carburetor.
- Carburetor Size (mm)
- Type of carburetor (Clamp-on, Spigot Mount, Flange Mount, Flat-slide, etc.)
- Application (Model, Year, Displacement)
- Modifications to motorcycle (Explain)
- Type of motorcycle use (Street, Road Race, Drag, Motocross, Enduro, etc.)
- Type of fuel used
- Climate Conditions (Altitude, etc.)
If you'd rather start from scratch then Pro-Flo has decent prices for carburetors and intake manifolds and also offers the excellent tuning manual that Mikuni produces.
Even replacing the air
filter with a higher flow unit will call for re-jetting the carburetor
so if you want to avoid any hassles just keep the stock units in good operating
condition. A good fuel filter, constant changing or cleaning of air filters
and draining the fuel from the carb when its out of service for lengthy
periods will all contribute to a smooth running scooter.
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LINKS
| Eric Gorrs Carburetor page | carburetor basics | Sudco carburetor | carburetor parts |
| Uni air filter | motorcycle service/repair | Pr-Flo carburetors | |
| SOHC owners club | Online scooter parts |
Back to the Ch 125/150/250 page or some of my other scooter pages are listed below