Constant Velocity carburetor
May/2009 - This page is undergoing a major revison and update but in the meantime a lightly edited version of the original page appears below

         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.

      A "constant velocity" carburetor (CV carb) is used on most four stroke scooters (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. Advantages are that the carburetor adapts nicely to altitude changes and has 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
  • 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.
Don't overlook  fuel tank ventilation and fuel shut-off valve operations.  All these items must be working properly to ensure fuel flow from the tank to the carburetor. Any air leaks or restrictions to fuel flow will result in problems.   Items such as new fuel filters and fuel line are inexpensive so replace them if in doubt. The Honda scooter carburetor has few external adjustments other than idle. If your scooter has suddenly started acting strange, disassembly and a general cleaning of the carburetor may be necessary especially if there is high mileage or the scooter has been sitting for a long time. 

  Before removing the carburetor from the scooter, drain the gasoline in the float bowl. There is a screw (flat headed screwdriver needed) towards the bottom of the carburetor( #15 in the diagram below). On my Elite 150, the screw faces towards the front of the scooter and there is a spigot at the very bottom of the carburetor which should be attached to a piece of tubing through which the gas will flow once you loosen the screw. Find the other end of that piece of tubing and put something there to catch the gas. There shouldn't be much.

Now that the fuel is drained you'll need to do two more things before removing the carburetor.
First, clean the outside of the carburetor by removing any dirt from the surrounding area. An old toothbrush is helpful with heavy layers of grime. Use a solvent (from dishsoap to carburetor cleaner) that will cut oil and grease.  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.

Before you  remove the airfilter housings, disconnect 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 showing where all the connections are. Taking several pictures with your camera is also helpful. 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. Every spring , bike shops are inundated with customers who forgot to drain the carburetor bowls and subsequently have hard starting or rough idleing. 

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.   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

Underside of carb with bowl and float removed
CV carburetor

                  Main jet and Emulsion tube/needle jet~ #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 shut-off valve hanging from it. When the float bowl fills up with gas, the float pushes on the shut-off 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.  I've also used one of the compressed air guns used for cleaning dust from computers.

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 bystarter /automatic choke.

The Fuel bystarter valve (the what?)

auto bystarterMost 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.  When the engine is first started. electricity is fed to the two wires leading from the valve. The electrical wires lead to a heating element which heats a solid element. The element expands with the heat and pushes the needle down. The needle eventually meets a small orfice and plugs it when fully heated . The blocked orfice stops the fuel enrichment circuit from operating (choke is then "off").    The needle takes time to fully extend, thus giving the scooter time to "warm up".
electric chokeThis is a diagram of the Electric choke or auto-bystarter from a Honda General service manual back in the 1980's. After Yamaha and Honda experimented with a few other methods, the Electric bystarter became the standard method of cold starting on Automatic scooters.  It's pretty well standard on all carbureted scooters, dirtbikes and buggys using the automatic engine. 

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 and doesn't idle when cold so the bystarter valve must be "broken".  The flaw in this reasoning is that if only 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 may be a 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.     

carburetor size chartAn 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.

Jetting a new carburetor correctly can take a lot of experimentation and anything that can take some of the guesswork out is well worth the money. A local motorcycle tuner with a selection of jets is also an option.

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.  There are also a lot of aftermarket CV carbs now available on the market for the GY6  and cn250 clone motors that should be adaptable.  You may have to change the wiring for the autochoke, disconnect the tube coming from the radiator and  buy a different throttle cable to complete the adaptation. 

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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