The Keihin constant velocity carburetor on your Honda CH250 should perform well with the standard recommended settings under average load, climatic, and barometric conditions. All constant velocity (CV) carburetors are designed to compensate for changes in altitude and barometric pressure. But even a slight modification to increase engine performance will require retuning of your CV carburetor. Air-box modifications or the use of an aftermarket exhaust system may require jetting changes. Optional main jets and slow jets are available for your scoot in a variety of sizes. The function of the carburetor is to atomize fuel and mix it with air in proper proportions to suit engine operating conditions. In operation, the carburetor meters gas into the fast moving air passing through it.  The atomized gas (a mist of liquid fuel) is then vaporized (changed from a liquid to a gas) by engine heat and the heat of compression to provide a uniform and efficiently combustible air/fuel mixture.

In theory, the perfect air/fuel ratio is 14.7 parts of air to one part of gas, by weight.  A uniform air/fuel ratio of this proportion allows the mixture to burn completely without leaving an excess of either fuel or air.  Rich (excessive fuel) or lean (excessive air) mixtures both result in loss of power.  An excessively lean mixture can also cause engine damage.  An intentionally rich mixture (from applying the electric choke) is used for starting because a cold engine reduces vaporization.  A throttle valve (carburetor slide) controls the amount of air/fuel mixture delivered to the engine, regulating the engine's power output.  When the throttle valve opening is increased, engine speed (rpm) also increases and air rushes through the carburetor bore at a greater rate. Unfortunately, the rate of fuel flow through a fixed jet does not increase proportionately with an increase in air speed through a fixed venturi.  At high speeds, the air/fuel mixture tends to become richer.  For this reason, it is desirable to vary the venturi size and meter the fuel flow to maintain correct air/fuel mixture ratios over a wide range of operating speeds.  This is achieved by using compensating jets and air-bleeds (air jets).  Each of the carburetor circuits affects the delivery of the air/fuel mixture over a given portion of the throttle valve opening. These circuits overlap.

Idle & Low Speed System

An adjustable pilot screw controls the idle mixture.  The pilot screw is located on the back of the carburetor in the passage between the low speed jet and the idle fuel discharge orifice to control the rate of flow of aerated fuel delivered to the carburetor bore. It has a limiter cap on it so adjustment is only slightly above three-quarters turn. An adjustable throttle stop screw controls the idle speed by raising or lowering the slide position when the throttle is closed.  A replaceable slow jet, located next to the main jet, controls the amount of fuel entering the idle and low speed system. The slow jet on the 1985-88 CH250 is #38. To fatten up the idle and slow speed circuit, it may be necessary to raise the slow jet to a #40 or #42 when the main jet is increased.

Intermediate System

Opening the throttle valve (carburetor slide) permits a transition from the low speed system to the intermediate system which meters fuel from the main fuel discharge (needle) jet.  A tapered fuel metering rod (jet needle) connected to the throttle slide, extends down into the main (needle) jet.  The jet needle position, which is adjustable, maintains the correct air/fuel mixture ratio through most of the carburetor's operating range, just short of fully open throttle. At that point the jet needle is fully raised, and fuel flow will be controlled primarily by the main jet. The main jet in the 1985 CH250 is a #115 and #112 in 1986-1988 as emissions requirements necessitated the leaner jet. Some modifications in this fuel delivery range have been beneficial. To increase the fuel flow at idle, some technicians place a thin brass washer (1mm, 1.5mm or 2mm) under the fat side of the jet needle which raises it at its rest position. Additionally, drilling out the vacuum port hole on the bottom of the throttle slide to 1/8" has increased the throttle response under load remarkably.

High Speed System

Fuel delivery is controlled by the size of the replaceable main (needle) jet and the thickness of the jet needle. While it is unnecessary to fatten the jet needle for anything other than competition performance, fattening the main jet from a #110 or #112 to a #115, #118 or #120 has shown significant improvement in the high speed range.

Float System

The float system is designed to maintain a constant and correct level of fuel in the carburetor's float bowl.  A float rises or falls with the fuel level in the float bowl.  The correct float bowl fuel level is established by Keihin.

Float System Vents

Float system vents are necessary to ensure a smooth flow of fuel through the carburetor.  The externally vented float bowl has its vent tubes routed to atmosphere so that atmospheric pressure can maintain pressure on the fuel inside the float bowl.  Vent tube routing is critical.  Any change in the stock routing of the tubes may pinch the tubes.  Improperly routed tubes may also be exposed to low pressure when the bike is in
motion, which could change the pressure in the float bowl and alter fuel delivery.

Air Metering Systems

Air under atmospheric pressure is bled, into the carburetor fuel passages to improve fuel atomization, to stabilize fuel height in the jets, and to provide corrections in the air/fuel mixture ratio.  Air jets and/or air-bleed adjustment screws control the relative amount of atmospheric air drawn into the fuel systems.  This system is factory pre-set and should not be altered.

Main Jet Air-Bleed System

Low venturi pressure, which causes fuel to rise through the main fuel jet, also causes atmospheric air to flow through the air jet. Air and fuel meet and mix together in a perforated (emulsion) tube above the main fuel jet.  The aerated fuel released into the venturi is more easily atomized than a dense un-aerated stream of fuel.  Aerated fuel also has less tendency to fall back down the jet tube between intake strokes, thus stabilizing fuel height in the jet tube.  The same effect can be observed when drinking beverages through a straw.  When you remove your mouth from the straw, a frothy beverage tends to remain in the straw, but an un-aerated beverage will fall back down the straw into the glass.

Air-Cut Valve

An air-cut valve is used to prevent popping in the exhaust system during deceleration.  The valve enriches the air/fuel mixture during deceleration.  A diaphragm in the air cut valve is activated whenever high manifold vacuum is present, such as during deceleration.  The movement of the diaphragm causes a partial blockage of the air bleed system in the low speed circuit.  This reduces the aeration to the low speed jet which creates a richer mixture.  The air-cut valve is factory pre-set and should not be altered.  It may have to be replaced eventually, because the rubber diaphragm in the valve may deteriorate over time.  If the diaphragm is deteriorating, you may notice leanness or a popping in the exhaust during deceleration.

Cold Starting System

Fuel does not vaporize well in a cold engine.  For this reason, the carburetor must deliver a richer mixture.  The mixture must not be excessively en-richened, however, or the combustion chamber can become flooded with liquid fuel.  The Honda CH250 uses an electric choke (autobystarter) that en-richens the mixture by obstructing the carburetor bore.  When the scoot is cold and the bystarter needle non-energized, it reduces the volume of air that can flow through the carburetor bore to fill the vacuum created in the engine cylinder. Atmospheric pressure in the float bowl then forces more fuel into the carburetor bore.

Keihin Jets--Mixing and Matching

The mid 1980s Hondas---CH125/150/250--all have the Keihin CV Constant Velocity carburetors. These carburetors are stamped "VB", "VD" or "VE" on the housing. Their function is similar. The only difference are jet sizes and plenum air flow chambers. The main jets are 8mm 393 series (thread 5x.75mm) an are numbered 99101-393-xxx (xxx=jet size). The slow or pilot jet is 23.5mm and either 26 or 420 series and are numbered N424-26-xxx (xxx= jet size). The lower the number the leaner the fuel mixture. Jets are available at your local Honda dealer or at rageperformance.com.
 

Year Model Main Jet Slow Jet 1984 CH125 #98 #35 1985 CH150 #95,98 #35 1985 CH250 #115 #38 1986 CH150 #100 #35 1986 CH250 #110,112 #38 1986 CN250 #110 #38 1987 CH150 #98,102 #35 1987 CH250 #110,112 #38 1987 CN250 #110 #38 1988 CH250 #110,112 #38 1989  CH250 #110,112 #38 1990 CH250 #110,112 #38 1992-on CN250 #108,110 #38

Also, any Honda Helix carburetor will also fit the Honda CH250. They have about the same dimensions---50mm OD on the air box side, 36mm OD on the engine side and 30mm ID on the engine side.


Randy Pozzi (Rev. 07/2006)