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====== The Keihin VD carburetor ======

{{ .:keihin:honda-ntv650-keihin-carburetor-code.jpg?direct&200|}}
Here some details about the carburetors mounted on the Honda NTV 650 Revere. They are two **Keihin VD** carburetors marked as **G4AACDt**. They are **constant velocity (CV) carburetors**, that uses engine vacuum to operate a diaphragm-controlled slide, rather than a direct cable connection.

===== Nomenclature =====


  * Jet needle
  * Main jet
  * Needle jet
  * Slow jet
  * Pilot screw
  * Float valve
  * Choke valve
  * Air cut off valve

**NOTICE**: The terms **slow**, **idle**, and **pilot** are used more or less interchangeably in Honda manuals and Keihin carburetor documentation.

{{.:keihin:keihin-side-view.jpg?180|}}
{{.:keihin:keihin-float-chamber.jpg?180|}}
{{.:keihin:keihin-vacuum-chamber.jpg?180|}}

{{.:keihin:keihin-airbox-view.jpg?180|}}
{{.:keihin:keihin-engine-view-bottom.jpg?180|}}
{{.:keihin:keihin-engine-view-top.jpg?180|}}

{{.:keihin:keihin-air-cut-off.jpg?180|}}
{{.:keihin:keihin-jets.jpg?180|}}
{{.:keihin:keihin-slide-diaphragm-needle.jpg?180|}}

{{.:keihin:keihin-float-bowl.jpg?180|}}

  * **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.

===== The pilot screw =====

The Pilot screw is on the engine side of the carb, located after the pilot jet, it is a fuel screw.

  * Turning it IN (tightening clockwise) reduces fuel flow, leaner mixture
    * Effects:
      * Harder cold start.
      * Lower idle speed.
      * Possible hesitation off idle.
      * Lean deceleration popping.
  * Turning it OUT (loosening counterclockwise) increases fuel flow, richer mixture
    * Effects:
      * Smoother idle.
      * Better throttle pickup.
      * Easier starting.
      * Reduced decel popping.

===== The air cut off valve =====

{{.:keihin:air-cut-off-valve_transient-enrichment-system.jpg?400|}}

===== Web References =====

  * **[[https://www.motorcycleproject.com/text/how_cv_carbs_work.html|How CV carburetors work]]** - {{:tecnica:motori:keihin:www.motorcycleproject.com-how-cv-carbs-work.pdf|Local PDF copy}}.
  * **[[https://gadgetsfixitpage.com/article.php/the-keihin-cvk-40-carburetor|The Keihin CVK-40 Carburetor]]** - {{ :tecnica:motori:keihin:care-and-feeding-of-the-keihin-carb.pdf|Local PDF copy}}.
  * **[[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"]]**