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tecnica:motori:keihin_vd_carburetor [2026/04/17 15:41] – [Fuel Supply System (Figure 4-15)] niccolotecnica:motori:keihin_vd_carburetor [2026/04/17 18:22] (current) – [Nomenclature] niccolo
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 {{.:keihin:keihin-slide-diaphragm-needle_annotated.jpg?480|Slide, diaphragm and needle}} {{.:keihin:keihin-slide-diaphragm-needle_annotated.jpg?480|Slide, diaphragm and needle}}
  
-{{.:keihin:keihin-float-bowl.jpg?480|Float bowl}}+{{.:keihin:keihin-float-bowl_annotated.jpg?480|Float bowl}}
  
 +===== Carburetor Functioning =====
  
 +Excerpts from the Service Manual of military motorcycle Harley Davidson MT-350.
  
-  * **Side view** 
-    * Air intake from the airbox. 
-    * Choke valve seat. 
-    * Pilot screw seat. 
-    * Duct for the high intake vacuum that acts on the //air cut-off valve// diaphragm. 
-    * //Air cut-off valve//: air in (normally open). 
-    * //Air cut-off valve//: air out. 
-  * **Float chamber** 
-    * Main jet 
-    * Slow jet 
-    * Starter jet (choke/enrichment jet) 
-    * Float valve 
-    * Breath hole 
-  * **Vacuum chamber** 
-    * Air intake from the airbox. 
-    * Starter (choke enrichment) air supply passage. 
-    * Air intake for the //air cut-off valve//: normally open. It closes on deceleration (high rpm + throttle closed). 
-    * Air intake for the slow jet. 
-    * Air intake for the choke. 
-    * Small vent that connects the chambers above and below the diaphragm: it acts as a vacuum/pressure control and damping system. 
-  * **Airbox view** 
-    * Air intake for the pilot and the //air cut-off valve// circuits. 
-  * **Engine view, bottom** 
-    * Primary pilot outlet port. This is the main idle fuel outlet, controlled by slow jet and pilot screw. 
-    * Pilot progression ports. They provide staged extra fuel during throttle opening. 
-  * **Engine view, top** 
-    * Starter (enricher) outlet port and vacuum pickup port that operates the //air cut-off valve//. 
-  * **Slide, diaphragm, needle** 
-    * Slide lift holes: vacuum bleed holes that control how quickly pressure changes reach the above diaphragm chamber. 
-  * **Float bowl** 
-    * Drain screw. 
-    * Drain conduct. 
  
-===== Functioning ===== 
  
 ==== Fuel Supply System (Figure 3-3) ==== ==== Fuel Supply System (Figure 3-3) ====
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 {{.:keihin:figure_3-3.png?400|Fuel Supply System}} {{.:keihin:figure_3-3.png?400|Fuel Supply System}}
  
-==== Starter System (Figure 4-16) ====+==== Starter System (Figure 3-4) ====
  
 The starting circuit consists of a cable actuated starter valve and converging fuel and air passages in the carburetor body. The starting circuit consists of a cable actuated starter valve and converging fuel and air passages in the carburetor body.
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 Low pressure, (vacuum) created by the downward stroke of the engine pistons, causes the higher pressure in the float chamber to force the fuel/air mixture through the fuel/air outlet passage in the carburetor venturi. Low pressure, (vacuum) created by the downward stroke of the engine pistons, causes the higher pressure in the float chamber to force the fuel/air mixture through the fuel/air outlet passage in the carburetor venturi.
  
-{{.:keihin:fig-4-16.png?direct&360|Starter System}}+{{.:keihin:figure_3-4.png?360|Starter System}}
  
-==== Idle and Low Speed Circuit (Figure 4-17) ====+==== Idle and Low Speed Circuit (Figure 3-5) ====
  
 At idle, with the throttle plate closed and the air stream cut off, idle speed is maintaned by fuel metered through the slow jet. Air from the slow air jet mixes with the fuel and is delivered to the idle port at the vacuum side of the throttle At idle, with the throttle plate closed and the air stream cut off, idle speed is maintaned by fuel metered through the slow jet. Air from the slow air jet mixes with the fuel and is delivered to the idle port at the vacuum side of the throttle
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 The venturi opening is reduced by the low positon of the vacuum piston. This enables initial air stream velocities to be higher than normaly attainable with fixed venturi carburetors. The higher air stream velocities provide greater quantities of fuel necessary for good acceleration. The venturi opening is reduced by the low positon of the vacuum piston. This enables initial air stream velocities to be higher than normaly attainable with fixed venturi carburetors. The higher air stream velocities provide greater quantities of fuel necessary for good acceleration.
  
-{{.:keihin:fig-4-17.png?direct&400|Idle and Slow Speed Circuit}}+{{.:keihin:figure_3-5.png?400|Idle and Slow Speed Circuit}}
  
-==== MidRange Slide Position and Fuel Discharge (Figure 4-18) ====+==== MidRange Slide Position and Fuel Discharge (Figure 3-6) ====
  
 As te throttle plate is opened, air flow increases through the carburetor and the pressure drop in the venturi near the needle jet increases. As te throttle plate is opened, air flow increases through the carburetor and the pressure drop in the venturi near the needle jet increases.
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 The tapered needle moves upward with the vacuum piston, opening the needle jet. The higher pressure in the float chamber forces fuel into the needle jet passage. Air at atmospheric pressure from the main air jet is forced through the main bleed tube opanings and mixes with the fuel. The fuel/air mixture is then delivered through the needle jet ino the air stream. The tapered needle moves upward with the vacuum piston, opening the needle jet. The higher pressure in the float chamber forces fuel into the needle jet passage. Air at atmospheric pressure from the main air jet is forced through the main bleed tube opanings and mixes with the fuel. The fuel/air mixture is then delivered through the needle jet ino the air stream.
  
-==== High Speed Circuit Slide Position and Fuel Discharge (Figure 4-19) ====+{{.:keihin:figure_3-6.png?400|MidRange Slide Position and Fuel Discharge}} 
 +==== High Speed Circuit Slide Position and Fuel Discharge (Figure 3-7) ====
  
 As the throtte plate is opened, the pressure difference betwneen the chambers above and below the diaphragm increases and the vacuum piston moves further upward. As the throtte plate is opened, the pressure difference betwneen the chambers above and below the diaphragm increases and the vacuum piston moves further upward.
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 The venturi opening increases and the needle is lifted further out of the the needle jet. The quantity of fuel and the volume of air are simultaneously increased and metered to the proportions of engine demand by the variable venturi and needle lift. With the vacuum piston fully upward, the venturi opening is fully enlarged and the needle jet opening exposure to the air stream is at its maximum, Air and fuel supplies are now available in quantities sufficient to meet all engine demands. The venturi opening increases and the needle is lifted further out of the the needle jet. The quantity of fuel and the volume of air are simultaneously increased and metered to the proportions of engine demand by the variable venturi and needle lift. With the vacuum piston fully upward, the venturi opening is fully enlarged and the needle jet opening exposure to the air stream is at its maximum, Air and fuel supplies are now available in quantities sufficient to meet all engine demands.
  
-==== Accelerator Pump System (Figure 4-19) ====+{{.:keihin:figure_3-7.png?400|High Speed Circuit Slide Position and Fuel Discharge}} 
 + 
 +===== The air cut off valve ===== 
 + 
 +{{.:keihin:air-cut-off-valve_transient-enrichment-system.png?480|}}
  
 ===== The pilot screw ===== ===== The pilot screw =====
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       * Easier starting.       * Easier starting.
       * Reduced decel popping.       * Reduced decel popping.
- 
-===== The air cut off valve ===== 
- 
-{{.:keihin:air-cut-off-valve_transient-enrichment-system.jpg?400|}} 
  
 ===== Web References ===== ===== Web References =====
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   * **[[https://www.venturerider.org/forum/forums/topic/65604-what-is-the-name-and-purpose-of-this-carburetor-part/|"Coasting enrichment valve" or "air cut-off valve" or "transient enrichment valve"]]**   * **[[https://www.venturerider.org/forum/forums/topic/65604-what-is-the-name-and-purpose-of-this-carburetor-part/|"Coasting enrichment valve" or "air cut-off valve" or "transient enrichment valve"]]**
   * **[[https://www.vessels-clan.com/jay/1200s/cv/|Keihin CV Carburetor Diagrams]]**   * **[[https://www.vessels-clan.com/jay/1200s/cv/|Keihin CV Carburetor Diagrams]]**
-  * **[[https://assets.publishing.service.gov.uk/media/5a81fe69ed915d74e623536d/2017-13198.pdf|Harley Davidson Manufacturer's Service Manual MT350 (84771039)]]**+  * **[[https://assets.publishing.service.gov.uk/media/5a81fe69ed915d74e623536d/2017-13198.pdf|Harley Davidson Manufacturer's Service Manual MT350 (84771039)]]** - {{.:keihin:harley-davidson--mt350-service-manual-84771039.pdf|Local PDF copy}}
  
tecnica/motori/keihin_vd_carburetor.1776433301.txt.gz · Last modified: by niccolo