The carburetor (ger. Vergaser) is an important mechanical part on motorcycle used for mixing air and fuel (gasoline) and atomizing it into explosive vapor needed for driving the engine. Further, carburetor's role is to automatically adjust the ratio of air and gasoline for appropriate combustion depending on driving and external atmospheric conditions.
Nowadays, in car industry almost without exception, and very often in motorcycle industry, for mixing of air and fuel an injection system is used (ger. Einspritz). But the carburetor is still often used on smaller engines and older motorcycles.
On Virago motorcycles the carburetor is mounted between two cylinders of V-twin engine. Smaller Virago models xv125 and xv250 do have one shared carburetor providing the same mixture for two cylinders while bigger models, from xv400 upwards, do have blocks of two carburetors, one carburetor per each cylinder.
Nowadays, in car industry almost without exception, and very often in motorcycle industry, for mixing of air and fuel an injection system is used (ger. Einspritz). But the carburetor is still often used on smaller engines and older motorcycles.
On Virago motorcycles the carburetor is mounted between two cylinders of V-twin engine. Smaller Virago models xv125 and xv250 do have one shared carburetor providing the same mixture for two cylinders while bigger models, from xv400 upwards, do have blocks of two carburetors, one carburetor per each cylinder.
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| Mikuni BST40 (sidedraft) on Yamaha xv750 Virago (75° V-twin) two carbs mounted perpendicular to bike driving axle |
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| Mikuni BDS34 (downdraft) on Yamaha xv535 Virago (70° V-twin) the block of two carbs between cylinders |
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| Mikuni BDS26 (downdraft) on Yamaha xv250 Virago (60° V-twin) one carb shared between two cylinders |
The most effective burning is achieved when there's the fuel-air mixture ratio of cca 1:14.7 (1:15). Carburetors are constructed to give richer mixture when starting the engine or during full power drive, and optimal mixture for driving at 60% to 70% of engine power.
In order to achieve efficient combustion, the fuel has to be atomized into smallest particles (atoms if possible) i.e. transformed into vapor (not mist with small gasoline drops). When air-fuel mixture comes into combustion chamber, the temperature and low pressure (vacuum) in cylinders help with process of vaporization. But when the engine is cold, not all fuel is fully vaporized and mixture becomes a mist still containing some quantity of small fuel drops which do not burn as well as vaporized gasoline and consequently additional gasoline has to be introduced, and for that the choke circuit is used.
In order to achieve efficient combustion, the fuel has to be atomized into smallest particles (atoms if possible) i.e. transformed into vapor (not mist with small gasoline drops). When air-fuel mixture comes into combustion chamber, the temperature and low pressure (vacuum) in cylinders help with process of vaporization. But when the engine is cold, not all fuel is fully vaporized and mixture becomes a mist still containing some quantity of small fuel drops which do not burn as well as vaporized gasoline and consequently additional gasoline has to be introduced, and for that the choke circuit is used.
Hitachi vs Mikuni
Early Virago models xv750, xv700, xv920, xv1000 and xv1100 since 1981. were using carburetors by Hitachi company. During the production of Virago motorcycles these carburetors undergone several revisions. For example in 1986. model xv1100 Virago was equipped with carburetor version with coasting enricher. The coasting enricher circuit makes richer mixture in pilot circuit by reducing air flow in pilot circuit (has air cut-off valve). Its purpose is to achieve better mixture burning and reduction of backfire in exhaust pipes often happening in motorcycle deceleration.
In 1987. Yamaha started to use carburetors by Mikuni, a Japanese company dedicated to carburetor production. Previously mentioned Virago models changed carburetors from Hitachi to Mikuni and Virago models xv535 from 1987., and xv250 which appeared even later, were using Mikuni carburetors from their beginnings.
In 1987. Yamaha started to use carburetors by Mikuni, a Japanese company dedicated to carburetor production. Previously mentioned Virago models changed carburetors from Hitachi to Mikuni and Virago models xv535 from 1987., and xv250 which appeared even later, were using Mikuni carburetors from their beginnings.
CV carburetors
On traditional carburetors (slide carburetors) a throttle cable is directly connected to throttle slider inside the carburetor controlling carburetor opening and the mixture flow directly and mechanically. When one pulls the throttle lever, the cable rises throttle slider, carburetor's Venturi tube is opened and bigger amount of air-fuel mixture instantly goes into carburetor and cylinders and the immediate acceleration occurs. The problem here is that fuel flow inside carburetor cannot always follow rapid increase of air arrival as a consequence of rapid opening of throttle slider and that results in lean mixture which disturbs proper engine operation.
Yamaha Virago models do use so called CV (constant velocity) or diaphragm type of carburetors. In these carburetors the throttle cable is not connected directly to throttle slider but to butterfly valve which sits back toward the engine side of the carb. It is opened and closed by means of the throttle cable and controls the amount of air that can flow through the carb. This carburetor type has two chambers (usually) separated by rubber diaphragm or membrane (could be cylinder too). The lower chamber is on external pressure, and the upper chamber is on low pressure (vacuum) generated by mixture (fluid) flow in main carb tube. When one pulls the throttle lever, that opens the butterfly valve resulting in increase of air flow. The higher flow in main tube results with lower pressure in main tube and the difference of pressures above and bellow diaphragm results with graduate movement of throttle slider and jet needle and that further results with new air-fuel mixture amount and ratio. So the throttle is controlled pneumatically not direct mechanically. Intention of CV is to keep the air flow constant.
Yamaha Virago models do use so called CV (constant velocity) or diaphragm type of carburetors. In these carburetors the throttle cable is not connected directly to throttle slider but to butterfly valve which sits back toward the engine side of the carb. It is opened and closed by means of the throttle cable and controls the amount of air that can flow through the carb. This carburetor type has two chambers (usually) separated by rubber diaphragm or membrane (could be cylinder too). The lower chamber is on external pressure, and the upper chamber is on low pressure (vacuum) generated by mixture (fluid) flow in main carb tube. When one pulls the throttle lever, that opens the butterfly valve resulting in increase of air flow. The higher flow in main tube results with lower pressure in main tube and the difference of pressures above and bellow diaphragm results with graduate movement of throttle slider and jet needle and that further results with new air-fuel mixture amount and ratio. So the throttle is controlled pneumatically not direct mechanically. Intention of CV is to keep the air flow constant.
In non-CV carburetors, when big amount of air comes too rapidly, the gasoline is not able to arrive so quickly and that results in lean mixture which furthermore causes the engine to stock and non-efficient combustion. The advantage of CV carburetor is that an increase of mixture flow is not happening so rapidly but gradually and CV carburetors always supply the engine with mixture quantity that the engine can exploit consequently achieving better efficiency and lower environment pollution. Yet another advantage of CV carburetor is that it is more resilient to influences of external pressure (altitude), air temperature, atmospheric conditions etc. because it operates on principle of pressure difference between main carburetor tube and pressure bellow diaphragm which is equal to atmospheric pressure. The drawback of CV carburetor is slower response on acceleration after pulling throttle lever.
Coasting enricher - air cut-off valve
Newer generations of carburetors do have an additional small diaphragm and spring to prevent afterburn generated by a lean fuel mixture when closing the throttle (during deceleration), the air cut-off valve temporarily closes the slow-jet passage, creating an instantly richer mixture.
Altitude dependency
Generally carburetors are tuned to work on altitudes up to 2.000 m (for us in Croatia that's fine). For higher altitudes some carburetor tuning should be done. Typical rule would be to use leaner mixture for higher altitudes and richer mixture for lower altitudes. That can be achieved by using appropriate main jet. For higher altitudes a main jet with smaller hole is needed in order to reduce the flow of gasoline into the mixture. Additionally the idle (pilot) mixture should be adjusted to be leaner on higher altitudes as well.
Construction types
Regarding the construction of air flow in carburetor there are three types of carburetors;
when air comes from bottom - updraft carb, when air comes from top - downdraft carb and when air comes from aside - sidedraft carb.
A typical CV carburetor does have air flow from aside (e.g. Mikuni BS34). These are so called natural or sidedraft carburetors. On smaller Viragos xv250 and xv535, the construction is made as such that air comes through a frame (under the tank) from above and therefore downdraft carburetors Mikuni BDS26 (diameter of 26 mm) and Mikuni BDS34 (diameter of 34 mm) are used. These are so called downdraft carburetors (D letter in name stands for downdraft). Bigger Virago models do use siderdraft carburetors with diameter of 40 mm, Mikuni BST40 and they are constructed perpendicular to bike frame (as left and right ones, not front and rear ones like BDS34 on xv535).
- Updraft - placed low on the engine and use a gravity fed-fuel supply - the tank is above the carburetor and the fuel falls to it. Even this carburetor uses gravity to receive the fuel from the tank, the air-fuel mixture must be forced upward into the engine.
- Downdraft - operates with lower air velocities and larger passages. This is because gravity assists the air-fuel mixture flow to the cylinder. The downdraft carburetor can provide large volumes of fuel when needed for high speed and high power output.
- Sidedraft - usual type used where there is little space on top of the engine. The air comes horizontally into the manifold.
A typical CV carburetor does have air flow from aside (e.g. Mikuni BS34). These are so called natural or sidedraft carburetors. On smaller Viragos xv250 and xv535, the construction is made as such that air comes through a frame (under the tank) from above and therefore downdraft carburetors Mikuni BDS26 (diameter of 26 mm) and Mikuni BDS34 (diameter of 34 mm) are used. These are so called downdraft carburetors (D letter in name stands for downdraft). Bigger Virago models do use siderdraft carburetors with diameter of 40 mm, Mikuni BST40 and they are constructed perpendicular to bike frame (as left and right ones, not front and rear ones like BDS34 on xv535).
How does a carburetor work you can find here.
| Yamaha xv750J Virago - Mikuni BST40 removal |
| Yamaha xv750J Virago - Mikuni BST40 disassembly |
| Code | xv125 | xv250 | xv400 | xv500 | xv535 | xv700 | xv750 | xv920 | xv1000 | xv1100 |
|---|---|---|---|---|---|---|---|---|---|---|
| 4RF-14901-00-00 | 97-00 | - | - | - | - | - | - | - | - | - |
| 3DM-14900-02-00 | - | 88-89 | - | - | - | - | - | - | - | - |
| 3DM-14900-03-00 | - | 89-92 | - | - | - | - | - | - | - | - |
| 3DM-14900-10-00 | - | 92-99 | - | - | - | - | - | - | - | - |
| 2UJ-14900-01-00 | - | 99-07 | - | - | - | - | - | - | - | - |
| 3BG-14900-01-00 | - | 99-07 | - | - | - | - | - | - | - | - |
| 3JB-14900-01-00 | - | - | 91 | - | - | - | - | - | - | - |
| 3JB-14900-10-00 | - | - | 92 | - | - | - | - | - | - | - |
| 3JB-14900-20-00 | - | - | 94 | - | - | - | - | - | - | - |
| 22U-14900-01-00 | - | - | - | 83-86 | - | - | - | - | - | - |
| 22U-14900-02-00 | - | - | - | 87 | - | - | - | - | - | - |
| 4FT-14900-00-00 | - | - | - | 92-93 | - | - | - | - | - | - |
| 4FT-14900-01-00 | - | - | - | 94 | - | - | - | - | - | - |
| 4FT-14900-20-00 | - | - | - | 95-96 | - | - | - | - | - | - |
| 4FT-14900-30-00 | - | - | - | 97-98 | - | - | - | - | - | - |
| 2GV-14900-00-00 | - | - | - | - | 87 | - | - | - | - | - |
| 2JV-14900-01-00 | - | - | - | - | 88 | - | - | - | - | - |
| 3BR-14900-00-00 | - | - | - | - | 88 | - | - | - | - | - |
| 3BR-14900-01-00 | - | - | - | - | 88 | - | - | - | - | - |
| 3BR-14900-10-00 | - | - | - | - | 89-91 | - | - | - | - | - |
| 3BT-14900-00-00 | - | - | - | - | 89-92 | - | - | - | - | - |
| 3BR-14900-11-00 | - | - | - | - | 92-93 | - | - | - | - | - |
| 3BT-14900-01-00 | - | - | - | - | 92-93 | - | - | - | - | - |
| 3BT-14900-02-00 | - | - | - | - | 94 | - | - | - | - | - |
| 3BT-14900-10-00 | - | - | - | - | 94 | - | - | - | - | - |
| 3BT-14900-20-00 | - | - | - | - | 94-96 | - | - | - | - | - |
| 3BT-14900-30-00 | - | - | - | - | 94-96 | - | - | - | - | - |
| 4KU-14900-00-00 | - | - | - | - | 96-97 | - | - | - | - | - |
| 4KU-14900-10-00 | - | - | - | - | 97-98 | - | - | - | - | - |
| 4KU-14900-11-00 | - | - | - | - | 98 | - | - | - | - | - |
| 4KU-14900-12-00 | - | - | - | - | 99-01 | - | - | - | - | - |
| 42W-14900-00-00 | - | - | - | - | - | 84-85 | - | - | - | - |
| 42W-14900-01-00 | - | - | - | - | - | 86-87 | - | - | - | - |
| 4X7-14900-01-00 | - | - | - | - | - | - | 81-83 | - | - | - |
| 55R-14900-00-00 | - | - | - | - | - | - | 84 | - | - | - |
| 1RW-14900-00-00 | - | - | - | - | - | - | 86 | - | - | - |
| 3AL-14900-01-00 | - | - | - | - | - | - | 88-94 | - | - | - |
| 3CM-14900-00-00 | - | - | - | - | - | - | 88-94 | - | - | - |
| 3AL-14900-02-00 | - | - | - | - | - | - | 95-98 | - | - | - |
| 3CM-14900-01-00 | - | - | - | - | - | - | 95-97 | - | - | - |
| 4PW-14900-00-00 | - | - | - | - | - | - | 95-96 | - | - | - |
| 5H1-14900-00-00 | - | - | - | - | - | - | - | 81-82 | - | - |
| 10L-14900-01-00 | - | - | - | - | - | - | - | 82-83 | - | - |
| 42G-14900-00-00 | - | - | - | - | - | - | - | - | 84-88 | - |
| 1TE-14900-00-00 | - | - | - | - | - | - | - | - | - | 86-87 |
| 3CF-14900-00-00 | - | - | - | - | - | - | - | - | - | 88-92 |
| 3CG-14900-00-00 | - | - | - | - | - | - | - | - | - | 88-92 |
| 3CF-14900-01-00 | - | - | - | - | - | - | - | - | - | 93-95 |
| 3CG-14900-01-00 | - | - | - | - | - | - | - | - | - | 96-99 |
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